System for optical wireless power supply

ABSTRACT

A system incorporating safety features, for optical power transmission to receivers, comprising an optical resonator having end reflectors and a gain medium, a driver supplying power to the gain medium, and controlling its small signal gain, a beam steering apparatus and a controller to control at least the beam steering apparatus and the driver. The controller responds to a safety risk occurring in the system, by outputting a command to change at least some of the small signal gain of the gain medium, the radiance of the optical beam, the power supplied by the driver, the scan speed or the scan direction and position of the beam steering apparatus, or to register the scan pose which defines the location of said optical-to-electrical power converter. The controller may also ensure a high overall radiance efficiency, and may warn of transmitted power not received by a targeted receiver.

FIELD OF THE INVENTION

The present invention relates to the field of wireless power beaming,especially as applied to use of a laser based transmission system tobeam optical power in a domestic environment to a mobile electronicdevice.

BACKGROUND

There exists a long felt need for the transmission of power to a remotelocation without the need for a physical wire connection. This need hasbecome important in the last few decades, with the popularization ofportable electronic devices operated by batteries, which need rechargingperiodically. Such mobile applications include mobile phones, laptops,cars, toys, wearable devices and hearing aids. Presently, the capacityof state of the art batteries and the typical battery use of a smartphone intensively used may be such that the battery may need chargingmore than once a day, such that the need for remote wireless batteryrecharging is important.

Battery technology has a long history, and is still developing. In 1748Benjamin Franklin described the first battery made of Leyden jars, thefirst electrical power source, which resembled a cannon battery (hencethe name battery). Later in 1800, Volta invented the copper zincbattery, which was significantly more portable. The first rechargeablebattery, the lead acid battery, was invented in 1859 by Gaston Planté.Since then the energy density of rechargeable batteries has increasedless than 8 times, as observed in FIG. 1, which shows the energydensity, both in weight and volume parameters, of various rechargeablebattery chemistries, from the original lead acid chemistry to thepresent day lithium based chemistries and the zinc-air chemistry. At thesame time the power consumed by portable electronic/electrical deviceshas reached a point where several full battery charges may need to bereplenished each day.

Almost a century after the invention of the battery, in the periodbetween 1870 and 1910, Tesla attempted the transmission of power overdistance using electromagnetic waves. Since then, many attempts havebeen made to transmit power safely to remote locations, which can becharacterized as over a distance significantly larger than thetransmitting or receiving device. This ranges from NASA, who conductedthe SHARP (Stationary High Altitude Relay Platform) project in the 1980sto Marin Soljacic, who experimented with Tesla-like systems in 2007.

Yet, to date, only three commercially available technologies allowtransfer of power to mobile devices safely without wires namely:

Magnetic induction—which is typically limited in range to just a few mm

Photovoltaic cells—which cannot produce more than 0.1 Watt for the sizerelevant to mobile phones when illuminated by either solar light or byavailable levels of artificial lighting in a normally (safe) lit room.

Energy harvesting techniques—which convert RF waves into usable energy,but cannot operate with more than 0.01 W in any currently practicalsituation, since RF signal transmission is limited due to health and FCCregulations.

At the same time, the typical battery of a portable electronic devicehas a capacity of between 1 and 100 Watt*hour, and typically requires adaily charge, hence a much higher power transfer at a much longer rangeis needed.

There is therefore an unmet need to safely transfer electrical power,over a large field of view and a range larger than a few meters, toportable electronic devices, which are typically equipped with arechargeable battery.

A few attempts to transfer power in residential environments, usingcollimated or essentially collimated, electromagnetic waves, especiallylaser beams, have been attempted. However, commercial availability ofsuch products to the mass market is limited at the current time. A fewproblems need to be solved before such a commercial system can belaunched:

-   -   A system should be developed which is safe.    -   A system should be developed which is cost effective.    -   A system should be developed which is capable of enduring the        hazards of a common household environment, including        contamination such as dust and fingerprints or liquid spills,        vibrations, blocking of the beam, unprofessional installation,        and periodic dropping onto the floor.

Currently allowed public exposure to transmitted laser power levels areinsufficient for providing useful amount of power without a complexsafety system. For example, in the US, the Code of Federal Regulations,title 21, volume 8, (21 CFR § 8), revised on April 2014, Chapter I,Subchapter J part 1040 deals with performance standards for lightemitting products, including laser products. For wavelengths outside ofthe visible range, there exist, class I, class III-b and class IV lasers(class II, IIa, and IIIa are for lasers between 400 nm and 710 nm, e.g.visible lasers). Of the lasers outside the visible range, class 1 isconsidered safe for general public use and classes IIIb and IV areconsidered unsafe.

Reference is now made to FIG. 2 which is a graph showing the MPE(maximal permissible exposure value) for a 7 mm. pupil diameter, forclass I lasers, according to the above referenced 21 CFR § 8, for 0.1-60seconds exposure. It can be seen from the above graph that:

(i) The maximum permissible exposure levels generally (but not always)increase with wavelength, and

(ii) Even if the laser is turned off some 0.1 second after a personenters the beam, in order to meet the requirement specified in 21 CFR §8, no more than 1.25 W of light can be transmitted, and that atwavelengths longer than 2.5μ, with the limit orders of magnitude less atshorter wavelengths.

Thus, without some kind of safety system, only a few milliwatts of laserpower are allowed to be transmitted, which even if completely convertedback to electricity, would supply significantly less power than thepower needed to charge most portable electronic devices. A cellularphone, for example, requires from 1 to 12 W for charging, depending onthe model.

To transmit power higher than that of class 1 laser MPE, a safety systemis needed. None, to the best of the applicants' knowledge, has yet beencommercialized for transmitting significant power levels in residentialenvironment accessible to untrained people.

Building of a transmission system having a robust safety system isdifficult. The required detection levels are very small compared to thepower that needs to be transmitted, the environment in which the systemoperates is uncontrolled and many unpredictable scenarios may happenwhile working.

It is well known in the art that fingerprints and dust scatter laserlight and that transparent surfaces reflect or scatter it. If high poweris to be transferred, then a class IV (or IIIb) laser would be needed,which would require a reliable safety system. For Class IV lasers, evenscattered radiation from the main beam is dangerous. According to the 21CFR § 8, as revised on April 2014, Chapter I, Subchapter J part 1040,lasers emitting between 400 nm and 1400 nm, having more than 0.5 W beamoutput, are usually considered class IV lasers for exposures above 0.5sec, and even scattered radiation from such lasers may be dangerous.Such lasers are required to have a lock key and a warning label similarto that shown in FIG. 3, where it is noted that the warning relates to“scattered radiation” also, and the user of the laser is usuallyrequired to wear safety googles and is typically a trained professional,all of these aspects being very different from the acceptable conditionsof use of a domestically available laser power transmission system forcharging mobile electronic devices.

The prior art typically uses anti-reflective coatings on surfaces toprevent such reflections, in combination with elaborate beam blockingstructures to block such reflections, should they nevertheless occur.However, the AR-coating solution used in the prior art is prone tofailure from dust or spilled liquid deposited on its surface, or fromcoating wear and tear, such as from improper cleaning. Additionally, thebeam block solution typically limits the field of view of the systemseverely, and is bulky compared to the dimensions of modern portableelectronic devices.

The prior art therefore lacks a reliable and “small footprint” mechanismto prevent scattering and reflections from the power beam in unwanteddirections. Such scattering and reflections may be caused either by atransparent surface inadvertently placed between the transmitter and thereceiver, and the optical characteristics of that transparent surfacemay arise from a vast number of different transparent materials, or fromliquid spills and fingerprints which may be deposited on the externalsurfaces of the system, typically on the front surface of the receiver.

A third problem with the solutions suggested in the prior art, is thatsuch safety systems generally require a mechanism to guarantee goodalignment of the power beam system and the safety system such that bothsystems are boresighted on the same axis until the power beam divergesenough or is attenuated enough (or a combination of these factors andany other factors) so that it no longer exceeds safety limits. This isextremely difficult to achieve with a collimated class IV or IIIb laserbeam, which typically expands very little with distance and thus exceedsthe safety limit for a very long distance.

One prior art principle of operation used to build such a safety systemis the optical detection of transparent surfaces that may be positionedin the beam's path. However transparent surfaces that may enter the beampath may be made from a vast number of different transparent materials,may be antireflection AR coated or may be placed in an angle close toBrewster's angle so they are almost invisible to an optical systemunless they absorb the beam. However, since light absorption levels foreach material are different, and may even be negligible, and sincebuilding an optical system that relies on optical absorption will behighly material specific, and since the number of available materials isextremely large, such a system is likely to be complex, large andexpensive, and unless properly designed, is likely to be unreliable,especially when considering that it is meant to be a critical safetysystem. Relying on the reflections to provide detectable attenuation ofthe beam is also problematic, as the surfaces may be coated by ananti-reflective coating or positioned in a near Brewster angle to thebeam, such that the reflection may be minimal for that particularposition of the surface.

Another limitation of prior art systems is that they are typically uselasers having good beam quality (low m2 value) combined with largeoptics in order to yield high efficiency (for example U.S. Pat. No.6,407,535 B1 and U.S. Pat. No. 6,534,705 B2 uses a wide aperture for thelaser beams) while U.S. Pat. No. 5,260,639 uses a wavelength of 0.8 umto allow for small optics, reducing cost and size of the optical system.

There therefore exists a need for a laser power transmission system withbuilt-in safety features, which overcomes at least some of thedisadvantages of prior art systems and methods.

The disclosures of each of the publications mentioned in this sectionand in other sections of the specification, are hereby incorporated byreference, each in its entirety.

SUMMARY

One of the main challenges of wireless power transmission is building asafe, low cost, and small transmitter and receiver, which isnevertheless powerful (e.g. capable of transmitting significant levelsof power). In order to enable powerful and small transmitter andreceivers it is essential to keep the radiance of the beam as high aspossible throughout the optical path, but especially at the output ofthe transmitter. Every component in the optical path causes a certainloss of radiance. In this document, the term radiance efficiency issometimes used—its usual meaning in the context of this document is theratio of the outgoing radiance of an optical component to the incomingradiance of the beam entering the component. For components that may beconfigured in various ways, such as a mirror that may be tilted ondifferent angles. There may be a different radiant efficiency for eachconfiguration.

In general the system as a whole needs to have a radiant efficiency ashigh as possible, typical values of more than 60% radiant efficiency andeven up to 90% or 95% should be strived for.

The transmitter's radiant efficiency is generally far more importantthan the receiver's radiant efficiency. There are two main factorsreducing the beams radiance—the laser system and the radiant efficiencyof the transmitter. Besides these factors, other more minor factors alsoinfluence it.

Lasers with high radiance values are generally bigger and more complex,while lasers having lower radiance are typically smaller and simpler.The present system is limited in laser radiance because it typicallyuses an uncommon wavelength allowing for improved safety features, andsmall, low cost, high radiance lasers at such unconventional wavelengthsare less commonly available and are likely to increase the cost of thesystem.

Prior art systems such as U.S. Pat. No. 5,260,639 and U.S. Pat. No.6,407,535 B1 use a shorter wavelength (0.8 μm or 0.532 μm) to allow fora more compact transmitter and receiver, however the current systemutilizes a longer wavelength and hence is required to use differentmethods to reduce the size of the system.

The longer wavelength used by the current system allows for detection oftransparent plastics in the beam's path by using a wavelength that isspecifically absorbed by virtually all plastic materials, as explainedin U.S. patent application Ser. No. 14/811,260, having common inventorswith the present application.

Opaque or even partially opaque materials can be easily detected whenplaced in the beam, by measuring the beam's attenuation. However somematerials are transparent or nearly transparent and it is suchtransparent materials that are significantly harder to detect. There aretwo major groups of solid transparent materials, organic and inorganicmaterials. The number of inorganic transparent solid materials availableto the general public is fairly limited, consisting mostly of glasses, afew semiconductor materials in common use, quartz, and some naturallyoccurring minerals such as diamonds, ruby and calcite. It is thereforepossible to build a detection system for reflections from inorganictransparent materials, covering all likely scenarios.

On the other hand, the availability of different organic, transparentmaterials to the general public is enormous, and new transparentmaterials are being added to the list all the time. This is asignificant problem as characterizing this group optically is thusvirtually impossible.

Polymers are a significant group of transparent organic materials, andthey will be used as a sample group to assist in explaining the way inwhich the current invention is intended to operate. Polymers typicallyconsist of long chains of monomers, with the backbone of such polymersbeing typically made of either carbon or silicon. FIGS. 4 to 9 show thechemical structure of some commonly used transparent polymers. FIG. 4shows a Poly-methyl methacrylate (PMMA) chain; FIG. 5 shows thestructure of a polycarbonate; FIG. 6 shows the polystyrene structure;FIG. 7 shows nylon 6,6; FIG. 8 shows a polypropylene chain; and FIG. 9shows the polyethylene chain structure.

As is observed, the chemical structure of the sample polymers shown isvery different, and the absorption spectra of these polymers depend onmany factors including the density of the material, trace amounts ofreagent, and the chain length. Yet it is observed that all the abovetransparent polymers have some chemical bonds in common, especially C—Cand C—H bonds. This is especially true for commercially availablepolymers, which are almost entirely based on organic materials, whichwould be detected by the systems of the present disclosure, orsemi-organic silicon based polymers such as silicones, polysilanes,polygermanes and polystannanes, or polyphospahazenes, which would alsobe detected by systems of the present disclosure.

Apart from that, the number of transparent materials available to thegeneral public which is not based on carbon chemistry is fairly limited,consisting mostly of various glasses, most of which have readilyavailable data on their transmission spectra.

If a system were designed so that the laser excites either a vibrationalC—H, or also possibly a C—C bond in polymers, then it would be easy todetect when one such polymer were positioned within the beam, bymonitoring the power drop caused by the polymer. This assumes that theabsorption of the C—H or C—C bond is always present and is alwayswavelength aligned to the laser wavelength. Rotational peaks could alsobe used for this purpose, but they may be unreliable in polymers, suchthat the vibrational C—H (or C—C) absorptions are better suited for thispurpose.

Reference is now made to FIG. 10, which shows a chart of typicalabsorption regions of different polymer bonds. It is observed that theC—H stretch vibration around 2900-3200 cm⁻¹, appears in almost all ofthe polymers shown. This could therefore be used as the absorptionmechanism trigger for a safety system, using the change in transmittedpower resulting from the absorption bands. However, there are twoproblems with these absorption bands, which make them less useful forthis purpose.

(i) The C—H vibrational absorption lines are typically very sharp, andtheir exact frequency varies much from one polymer to another, so alaser may excite one polymer, but not another. Thus, unless the laser istuned exactly to the specific C—H vibration line of that polymer, itwould not be absorbed.

(ii) Such C—H vibration peaks are generally medium absorption peaks,meaning that the attenuation of a beam due to a material section a fewmm thick would be 20-50% (i.e. it allows detection of even trace amountsof material in a small container), and while medium (20-70% attenuationper cm material) and strong (>70% attenuation per cm) absorption peaksare generally much easier to detect, they cannot be used to construct arobust system.

In a commercial system, designed for the consumer environment,fingerprints are a common problem. In normal operation the system shouldnot fail simply when a fingerprint is deposited upon it; instead thesystem should shut down transmission when there is a risk of exceedingsafety limits. To do this, the system should detect blocking of the beambut should not cease transmission due to any fingerprint deposited onthe receiver. If a strong or medium absorption peak is used, then shoulda fingerprint or some other contamination be deposited on the externaloptical surface of the receiver or transmitter, it would absorb the beamsignificantly, causing power transmission to fail. This arises sincefingerprints also contain organic compounds that would absorb the beam,resulting in uncontrolled system failure. In order to allow the systemto operate in an environment where organic materials such asfingerprints may be deposited on the surface of its typically externaloptical components, it would be necessary to build a system where thelaser beam successfully traverses the finger print, while the safetysystem detects dangerous transparent items that may be inserted into thebeam. If, on the other hand, a safety system were to utilize a weakabsorption band instead of a medium or strong one, then the systemshould continue to operate with the fingerprint, and shutoff may be donebased on an electronic decision and not in an uncontrolled manner.

Turning to the C—C absorption band, stretching from 800 cm⁻¹ to 1300cm⁻¹, this is such a wide band that a narrowband laser is almost certainto miss a narrowband absorption peak in this region, since while thepeak may be positioned in the 800 cm⁻¹ to 1300 cm⁻¹ range, its typicalwidth is very small, and may be easily missed by a narrowband laser.Additionally, as will be seen in FIG. 11 hereinbelow, this band vanishesfor some polymers, where no absorption peak is visible between 800 and1300 cm⁻¹ and some polymers may exist where C—C bonds are not present,and are replaced by aromatic carbon-carbon bonds or by C═C bonds andC—O—C bonds

An additional problem arises from the absorption strength of the C—Cline. In symmetrical compounds such as polyethylene, it may be nearlyimpossible to detect, while in other compounds it may be so strong thateven a weak fingerprint on the surface of the receiver will make thesystem inoperable, as a significant portion of the power may be absorbedby the fingerprint, making the device unusable. To enable operation of asystem in which fingerprints may be deposited on its optical surfaces, aweak, but not too weak absorption line is required that will not changemuch between different polymers and which would be found in most organicpolymers, and a laser tuned to that peak should be used, in conjunctionwith a system that operates around that peak. As can be seen from FIG.10, there is no such peak in the commonly used polymers and in theabsorption bands shown.

A system for optical wireless power transmission to a power receivingapparatus may comprise:

(a) an optical resonator having end reflectors and adapted to emit anoptical beam,

(b) a gain medium positioned inside the optical resonator and having afirst bandgap energy, the gain medium being thermally attached to acooling system and configured to amplify light passing through it,

(c) a collimating lens, reducing the divergence of the light, and havinga high radiant efficiency (>50%)

(d) a driver supplying power to the gain medium, and controlling thesmall signal gain of the gain medium,

(e) a beam steering apparatus configured to direct the optical beam inat least one of a plurality of directions, and typically having a highradiant efficiency, typically larger than 50%),

(f) an optical-to-electrical power converter configured to convert theoptical beam into electrical power having a voltage, theoptical-to-electrical power converter having a second bandgap energy,and a thickness such that it acts as an absorbing layer (typically asemiconductor).

(g) an electrical voltage converter, adapted to convert the voltage ofthe electrical power generated by the optical-to-electrical powerconverter into a different voltage, the electrical voltage convertercomprising an inductor, an energy storage device and a switch,

(h) at least one surface associated with the optical-to-electrical powerconverter and optically disposed between the gain medium and theoptical-to-electrical power converter,

(i) a detector configured to provide a signal indicative of the opticalbeam impinging on the optical-to-electrical power converter,

(j) a safety system assessing the potential for a breach of security,

(k) a controller adapted to control at least one of the status of thebeam steering apparatus and the driver, the controller receiving acontrol input signal from at least the detector, wherein:

(l) the at least one surface has properties such that it reflects asmall part of light incident on it, either (i) in more than onedirection, or (ii) such that the reflected light has a virtual focuspositioned remotely from the optical resonator relative to the surface,or (iii) such that the reflected light has a real focus positioned atleast 1 cm. in the direction of the optical resonator relative to thesurface,

(m) the controller is configured to respond to the control input signalreceived from the detector by at least one of (i) causing the driver tochange the small signal gain of the gain medium, (ii) changing theradiance of the optical beam, (iii) changing the power supplied by thedriver, (iv) changing the scan speed of the beam steering apparatus, (v)changing the scan position of the beam steering apparatus, and (vi)recording a scan position defining the position of theoptical-to-electrical power converter,

(n) the gain medium is a semiconductor device or a solid host doped withNd ions, and includes a filter attenuating radiation for at least onefrequency having a wave number in the range 8,300 cm⁻¹ to 12,500 cm⁻¹,

(o) the thickness of the optical-to-electrical power converter activesemiconductor layer is selected to be large enough to absorb most of theoptical beam yet not so large that the quantum efficiency of thesemiconductor layer becomes significantly reduced.

(p) the second bandgap energy is smaller than the first bandgap energy,

(q) the first bandgap energy is between 0.8 eV and 1.1 eV,

(r) the switch has a closed serial resistance smaller than R, given bythe equation

$R \leq \frac{{E_{gain}}^{2}}{2*10^{- 40}*P_{{laser}\mspace{14mu} {driver}}}$

where R is measured in Ohms, E_(gain) is the first bandgap energymeasured in Joules, and P_(laser driver) is the power supplied by thelaser driver to the gain medium, measured in Watts, (s) the optical beamhas a radiance of at least 8 kW/m²/Steradian, and a frequency betweenthe first overtone of the C—H absorption situated at approximately 6940cm⁻¹ and the second overtone of the C—H absorption situated atapproximately 8130 cm⁻¹, and (t) the optical components in the system,especially the collimation lens and the beam director, have a radiantefficiency of at least 50%.

In any such a system, the different voltage may be a higher voltage thanthe voltage generated by the optical-to-electrical converter.Furthermore, the status of the beam steering apparatus may be either orboth of the aiming direction and the scan speed of the beam steeringapparatus.

Furthermore, in any of the above-described systems the optical beam mayhave a radiance of at least 800 kW/m²/Steradian.

Another example implementation can involve any of the above describedsystems in which each one of the end reflectors of the resonator areeither (i) dielectric mirrors, (ii) Bragg mirrors, (iii) Fresnelreflectors or (iv) mirrors composed of alternating layers of dielectricor semiconductor material having different refractive indexes.Additionally, the gain medium can be either a transparent solid hostmaterial doped with Nd ions or a semiconductor. In such a case, thesystem may further comprise a filter for extracting radiation having awave-number greater than 8300 cm⁻¹. In the event that the gain medium isa semiconductor, it may advantageously be a quantum dot gain medium.

In further exemplary implementations of the above described systems, thecooling system may be at least one of a heatsink, a Peltier diode, and aliquid cooled plate. It may also be equipped with a fan. Additionally,the gain medium may be attached to the cooling system using a layer ofsolder having less than 200° Kelvin/Watt thermal resistance. In anyevent, the cooling system may be such that the thermal resistancebetween the gain medium and the surrounding air is less than 200°Kelvin/Watt.

In alternative implementations of any of the above-described systems,the optical-to-electrical power converter may be a photovoltaic cell. Insuch a case, the photovoltaic cell may be a III-V device. In any event,the serial resistance of the optical-to-electrical power convertershould be less than 1 Ohm.

The optical to electrical power converter typically has conductors onit, which have a thickness of at least 0.02/μ₁₀ where μ₁₀ is the decadicattenuation coefficient measured in 1/m.

Such conductors should have a thickness that is at least

$\frac{0.01*P\; \rho}{V^{2}*\chi}$

meters,

where P is the transmitted power absorbed by the photovoltaic, measuredin watts,

ρ is the specific electrical resistivity of the conductors,

V is the voltage emitted by the photovoltaic cell at its maximal powerpoint, and

χ is the fraction of the area of the absorbing layer covered byconductors

According to further implementations of the above described systems, theinductor should have a serial resistance measured in Ohms of less thanthe square of the first bandgap energy measured in Joules divided by2*10⁻⁴⁰ times the driver power measured in Watts. In otherimplementations, the energy storage device may be either a capacitor ora rechargeable battery.

According to further implementations of the above described systems, atleast one safety system is included, which estimates the probability ofa breach of safety, given inputs from various sensors and monitors Thisis different from prior art systems, which provide actual measurementdata only, such as the radar system in U.S. Pat. No. 6,407,535, with noindication of the probability of an error. The present system differs inthat it provides a signal indicative of the probability of a breach ofsafety, as opposed to an actually detected breach of safety. This allowsseveral significant advantages. Firstly, the system can react topotentially problematic situations, which become apparent by a lowsignal-to-noise, or a signal interruption, by differentiating betweenhigh risk situations and lower risk situations, and reacting differentlyto each situation. For instance, a low risk situation such as caused bya dirty aperture, by alignment faults, or similar occurrences, can bedealt with differently from a high risk situation, such as a highprobability of beam intrusion, or an unreasonable beam power, whetherhigh or low. Secondly, the system can combine probabilities fromdifferent safety systems into a unified probability, in order to achievea sufficiently high detection accuracy. For example, if a system isdesigned to have a failure rate of 10⁻⁹ failures per hour, typically, ina changing environment, no single safety system can provide suchreliable measurements without failure. However, a combination of safetysystems can have better failure rate and if such data is combined withthe probability of error, and if the statistical correlation of errorsfrom both safety systems is known or can be estimated or approximated,then the data from the two systems may be combined to yield data withsignificantly higher probability. Such reliability data may beestimated, inter alia, from signal-to-noise, from the temperature ofcomponents, from preloaded data based on measurements on the same deviceor on a similar device, from user entered information, updated by themanufacturer or distributer or supplied by the user.

According to further implementations of the above described system theoutput beam of said laser resonator is collimated (or brought to nearcollimation) on at least one axis using a lens. The lens needs to have ahigh radiance efficiency (typically more than 50%), for which a highNumerical Aperture (NA) lens should be used.

According to a further implementation of the above described systems,the beam deflection mechanism also needs to have a high radianceefficiency of more than 50% and also needs to be positioned so that itscenter of rotation is close to either the weighted average point of thebeam, to the maximal intensity point of the beam or to the center of the50% intensity line or 90% intensity lines of the beam.

An overall radiance efficiency for the transmission/reception/conversionprocess of 30% is a desirable level to render the system energyefficient, but it is to be understood that this is limited by theconstraints of available components, and their environmental state, andthat levels below 30% are also operable, such as 20%, or even less.

Additionally, any of the above described systems may further comprise aretro reflector. Also, the gain medium may be pumped electrically oroptically by the driver. Furthermore, the second bandgap energy may bemore than 50% of the first bandgap energy.

Yet other implementations perform a method for transmitting power from atransmitter to a receiver, comprising:

(a) converting a first electrical power to an electromagnetic wavehaving a frequency between the first overtone of the C—H absorptionsituated at approximately 6940 cm⁻¹ and the second overtone of the C—Habsorption situated at approximately 8130 cm⁻¹, the electromagnetic wavehaving a radiance of at least 8 kW/m²/Steradian, the converting beingperformed by using an optical resonator having end reflectors and a gainmedium connected to a laser driver receiving the first electrical power,the gain medium having a first bandgap energy between 0.8 eV and 1.1 eV,being positioned inside the optical resonator, being thermally attachedto a cooling system, and configured to amplify the electromagnetic wavepassing through it,

(b) directing the electromagnetic wave into at least one of a pluralityof directions using a beam steering apparatus controlled by acontrolling unit,

(c) detecting the impingement of the beam on a target having anassociated partially transparent surface, such that an indicationrelating to the impingement may be utilized by the controlling unit toperform at least one of (i) causing a change in the small signal gain ofthe gain medium, (ii) causing a change in the radiance of theelectromagnetic beam, (iii) causing a change in the first electricalpower, (iv) changing the scan speed of the beam steering apparatus, (v)changing the scan position of the beam steering apparatus, and (vi)recording a scan position defining the position of the target,

(d) converting the electromagnetic wave into a second electrical powerhaving a voltage, by using an optical-to-electrical power converterhaving a second bandgap energy smaller than the first bandgap energy,

(e) converting the voltage into a different voltage using an electricalvoltage converter, comprising an inductor, an energy storage device anda switch having a closed serial resistance smaller than R, given by theequation

${R \leq \frac{{E_{gain}}^{2}}{2*10^{- 40}*P_{{laser}\mspace{14mu} {driver}}}},$

where R is measured in Ohms, E_(gain) is the first bandgap energymeasured in Joules, and P_(laser) _(_) _(driver) is the first electricalpower, measured in Watts, wherein

(f) the surface is designed such that it reflects a small part of theelectromagnetic wave incident on it either (i) diffusively, or (ii) suchthat the reflected light has a virtual focus positioned remotely fromthe optical resonator relative to the surface, or (iii) such that thereflected light has a real focus positioned at least 1 cm. in thedirection of the optical resonator relative to the surface, and

(g) the gain medium is either a semiconductor device, or a solid hostdoped with Nd ions that includes a filter attenuating radiation for atleast one frequency having a wave number in the range 8,300 cm⁻¹ to12,500 cm⁻¹,

In such a method, the switch may be switched at a frequency determinedby the equations

$f < {\frac{1}{1.28*10^{- 40}*L}*{E_{gain}}^{2}*\frac{1 - \frac{E_{gain}}{5*10^{- 19}*V_{output}}}{P_{{laser}\_ {driver}}}}$$f > {\frac{1}{3*10^{- 38}*L}*{E_{gain}}^{2}*\frac{\left( {1 - \frac{E_{gain}}{4*10^{- 20}*V_{output}}} \right)}{P_{{laser}\_ {driver}}}}$

where f is the switching frequency measured in Hz., E_(gain) is thebandgap of the gain medium, measured in Joules, V_(output) is the outputvoltage from the voltage converter, measured in Volts, andP_(laser driver) is the power pumped by the laser driver onto the gainmedium, measured in Watts.

Additionally, the detection of impingement of the beam on the target maybe done using either detection in the transmitter of retro reflectedillumination from the target, or detection of illumination of the targetusing a receiver sensor.

Furthermore, in any of the above described methods, the second bandgapenergy may be more than 50% of the first bandgap energy.

There is thus provided in accordance with an exemplary implementation ofthe devices described in this disclosure, a system for optical wirelesspower transmission to at least one power receiving apparatus, the systemcomprising:

(i) an optical resonator having end reflectors and a gain medium, thegain medium comprising either a semiconductor material, or a solid host,doped with Neodymium ions and in optical communication with a filterattenuating radiation for at least one frequency having a wave number inthe range 8,300 cm⁻¹ to 12,500 cm⁻¹, and wherein the gain medium (a) ispositioned inside the optical resonator, (b) has a first bandgap energy,and (c) is thermally attached to a cooling system, such that the gainmedium is configured to amplify light passing through it, and theresonator is configured to emit an optical beam of the light,

(ii) a driver configured to supply power to the gain medium, andenabling control of the small signal gain of the gain medium,

(iii) a beam steering apparatus configured to direct the optical beam inat least one of a plurality of directions,

(iv) an optical-to-electrical power converter located in the at leastone power receiving apparatus, and configured to convert the opticalbeam into electrical power having a voltage, the optical-to-electricalpower converter having a second bandgap energy,

(v) a detector configured to provide a signal indicative of the opticalbeam impinging on the optical-to-electrical power converter, and

(vi) a controller adapted to control at least one of the status of thebeam steering apparatus and the driver, the controller receiving acontrol input signal at least from the detector, wherein the controlleris configured to respond to an indication of a safety risk occurring inthe system, by outputting a command to result in at least one of:

-   -   causing the driver to change the small signal gain of the gain        medium,    -   changing the radiance of the optical beam,    -   changing the power supplied by the driver,    -   changing the scan speed of the beam steering apparatus,    -   changing the scan pose of the beam steering apparatus, and    -   recording the scan pose which defines the location of the        optical-to-electrical power converter.

The term pose is understood to mean both the position and the angularorientation in which the beam steering apparatus directs the beam.Furthermore, the above-mentioned driver configured to supply power tothe gain medium, is also understood to be able to control of the smallsignal gain of the gain medium either by varying the pump power input tothe gain medium, or even by turning the driver completely on or off.

In such a system, the indication of a safety risk occurring in thesystem may be obtained at least from the signal generated by thedetector configured to provide a signal indicative of the optical beamimpinging on the optical-to-electrical power converter, and from asignal generated by the level received at the resonator of the beamreflected from the at least one power receiving apparatus.

In many situations, it is necessary to limit the maximal power emittedby the system, to prevent exceeding some maximal value which may bederived from safety requirements of the system, from engineeringrequirements of the system, from the receiver's power requirements(which may dynamically change) or from other issues. In such a case,when the optical beam exceeds a certain power the system may reduce thesmall signal gain of the gain medium which will result in lowering ofthe radiance emitted by the system.

Consequently, according to yet further implementations, such systems mayfurther include a power sensor disposed such that it provides a signalindicative of the power carried by the optical beam before impingementon the at least one power receiving apparatus. In such a situation, thedriver may be configured to reduce the small signal gain of the gainmedium when the power indication of the power sensor exceeds athreshold.

Additionally, one important indication of safety risk is “lost power”,an estimation of the power unaccounted for by the system. Such power maybe lost to system inefficiency but may also be lost to power emission ina risky manner. If such “lost power’ is detected, the system shouldperform various operations to ensure safe operation, which may includereducing the beam's power, reducing the small signal gain, reducing thesystem's radiance, diverting the beam or informing the user. Therefore,the detector may also provide a signal indicative of the power receivedby the at least one power receiving apparatus. In this case, at leastone of the indications of safety may come from a difference between thepower indicated by the power sensor and the power indicated by thedetector in one of the at least one power receiving apparatus. At leastone of the indications of safety may then arise from the differenceexceeding a threshold.

Other safety hazard indications may come from a beam penetration sensor,which may be optical, or from system integrity sensors such aswatchdogs, interlocks, thermistors, which may indicate that the systemis not safe. In such a case, the system may perform a safety operationsuch as reducing the beam's power, reducing the small signal gain,reducing the system's radiance, diverting the beam or informing theuser.

Therefore, further implementations of the above-described systems mayinclude a beam penetration sensor adapted to sense when an unwantedobject enters the optical beam, the entry of the unwanted objectconstituting an indication of a safety risk. Alternatively andadditionally, such systems may further include an enclosure integritysensor, wherein a warning issued by the sensor of lack of integrity ofthe enclosure indicates a safety risk. Such systems may also include asensing device for sensing a deviant operation of at least one criticalsubsystem in the system, the deviant operation constituting anindication of a safety risk.

Other exemplary systems for optical wireless power transmission to atleast one power receiving apparatus may comprise:

(i) an optical resonator having end reflectors and a gain medium, thegain medium comprising either a semiconductor material, or a solid host,doped with Neodymium ions and in optical communication with a filterattenuating radiation for at least one frequency having a wave number inthe range 8,300 cm⁻¹ to 12,500 cm⁻¹, and wherein the gain medium ispositioned inside the optical resonator, has a first bandgap energy, andis thermally attached to a cooling system, such that the gain medium isconfigured to amplify light passing through it, and the resonator isconfigured to emit an optical beam of the light,

(ii) a driver configured to supply power to the gain medium, andenabling control of the small signal gain of the gain medium,

(iii) a beam steering apparatus configured to direct the optical beam inat least one of a plurality of directions,

(iv) an optical-to-electrical power converter located in the at leastone power receiving apparatus, and configured to convert the opticalbeam into electrical power having a voltage, the optical-to-electricalpower converter having a second bandgap energy,

(v) a detector configured to provide a signal indicative of the opticalbeam impinging on the optical-to-electrical power converter, and

-   -   (vi) a controller adapted to control at least one of the status        of the beam steering apparatus and the driver, the controller        receiving a control input signal at least from the detector,        such that the optical beam has a radiance of at least 8        kW/m²/Steradian, and when the controller directs the optical        beam onto one of the power receiving apparatuses, the overall        radiance efficiency of transmission is at least 20%. In such a        system, the overall radiance efficiency of the transmission may        alternatively be at least 30%.

In many cases, designing a system to have high radiance may becomedifficult across dynamic temperatures and mechanical stresses. Often theoptical components in the beam path need to be aligned precisely, andchanging temperatures or mechanical stresses cause beam aberrations suchas defocus, comma and also misalignment. The system should be designedto either prevent such a change or to correct it by means of sensing thechanged radiance and adapting a parameter, such as lens position or beampower to improve it. Radiance measurement is also important for safetyreasons. Loss of radiance between the transmitter and receiver isusually indicative either of poor operational state of the transmitter(either the settings are not correct, or some optical, mechanical orelectronic component has gone out of alignment or is not working asexpected) or of a foreign object between the transmitter and thereceiver. A suitable response to such a situation may be to perform asafety operation such as reducing the beam's power, reducing the smallsignal gain, reducing the system's radiance, diverting the beam orinforming the user

Therefore, either of the latter two described systems may be configuredto maintain the overall radiance efficiency of transmission across apredetermined range of operational temperatures. They may furtherinclude a radiance detector in the at least one power receivingapparatus, providing a signal indicative of the radiance received by thepower receiving apparatus. In such a case, the overall radianceefficiency may be maintained because of at least one of (i)constructural parameters of the system, and (ii) the provision of aradiance maintaining feedback system, adapted to use the signalindicative the radiance and to adapt system parameters to maintain it.The system parameters may include at least one of a lens position, alens optical power and the beam power.

Furthermore, such systems may further include a transmission radiancedetector disposed to provide a signal indicative of the radiance emittedby the optical wireless power transmission system. In that case, anindication of a safety risk may be triggered when the difference betweenthe signal from the transmission radiance detector and the signalindicative of the radiance received by the power receiving apparatusexceeds a threshold value.

In any of the above-described systems, the second bandgap energy shouldbe smaller than the first bandgap energy.

Finally, in such systems, the indication of a safety risk occurring maybe further obtained when the difference between a signal indicative ofthe transmission radiance and a signal indicative of the radiancereceived by the power receiving apparatus exceeds a threshold value.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully fromthe following detailed description, taken in conjunction with thedrawings in which:

FIG. 1 shows the energy density of various battery chemistries;

FIG. 2 shows the maximal permissible exposure value for lasers forvarious exposure times, according to the US Code of Federal Regulations,title 21, volume 8, (21 CFR § 8), revised on April 2014, Chapter I,Subchapter J part 1040;

FIG. 3 shows an example of a warning sign for a class IV laser product;

FIGS. 4-9 show examples of the chemical composition of various commonlyused transparent polymers;

FIG. 4 shows a Poly-methyl methacrylate (PMMA) chain;

FIG. 5 shows the structure of a polycarbonate;

FIG. 6 shows the polystyrene structure;

FIG. 7 shows the structure of nylon 6,6;

FIG. 8 shows a polypropylene chain structure;

FIG. 9 shows the polyethylene chain structure;

FIG. 10 shows the IR absorption bands for common organic chemical bonds;

FIG. 11 shows the IR absorption spectrum of polyethylene;

FIG. 12 shows the overtone absorption bands for some common organicchemical bonds;

FIGS. 13a and 13b show different electronic configurations forconverting the output voltage of a photovoltaic cell to a differentvoltage;

FIG. 14 shows the power reflected per square meter by a mirror, when abeam of radiance 8 kW/m²/steradian is focused upon it, as a function ofnumerical aperture;

FIGS. 15A to 15C show schematic drawings of exemplary apparatusaccording to the present disclosure, for avoiding unsafe reflectionsfrom the front surface of a receiver being illuminated by a transmitterof the present disclosure;

FIG. 16 shows a schematic diagram showing a more detailed description ofthe complete optical wireless power supply system of the presentdisclosure;

FIG. 17 is a graph showing the change in power transmission of thesystem of FIG. 16, as a function of the angle of tilt of the beamsteering mirror;

FIG. 18 shows a schematic representation of a cooling system for thegain medium of the system of FIG. 16;

FIG. 19 is a schematic diagram showing a detailed description of thesystem of FIG. 16, but further incorporating a safety system;

FIG. 20 is a schematic view of the optical-to-electrical power converterof the systems shown in FIGS. 16, 19;

FIG. 21 shows a block diagram view of the safety system of FIG. 19;

FIG. 22 shows an output laser beam of the system of FIG. 19, deflectedby a mirror rotating on a gimbaled axis, or on gimbaled axes;

FIG. 23 shows the mirror of FIG. 22 rotated so that the beam isdeflected at a larger angle than that shown in FIG. 22;

FIG. 24 shows a schematic representation of the intensity profiles of atypical deflected beam;

FIG. 25 shows a side view of a laser diode from a directionperpendicular to the fast axis of the laser, and a lens for manipulatingthe beam; and

FIG. 26 shows a block diagram of the complete laser protector

DETAILED DESCRIPTION

In view of the above described considerations, one exemplaryimplementation of the optical wireless power supply systems of thepresent disclosure could be a system tuned to work in between the firstovertone of the C—H absorption at 6940 cm⁻¹ and the second overtone ofthe C—H absorption at 8130 cm⁻¹. Such overtone bands are less knownbands, containing much less chemical information, and arise fromessentially forbidden quantum mechanical transitions, and are onlyallowed due to complex mechanisms. Consequently, they provide wide, weakabsorption bands, exactly as preferred for this application, but havefound significantly less use in analytical chemistry. The broad natureof the bands allows for detecting various different polymercompositions, while the weak absorption allows the system to continueoperation even in the vicinity of organic dirt and fingerprints. Thismakes these lines significantly less useful for typical uses ofabsorption measurements, but ideal for the present task. Anotheradvantage of these lines is that there are no commonplace absorptionlines directly positioned at the same frequencies, so that changingchemical composition of the materials will not alter the measurementresults strongly. Many such overtone bands are illustrated in the chartof FIG. 12.

Electro-optical components that operate in that band are scarce and hardto source, probably since both diode lasers and diode-pumped, solidstate (DPSS) lasers are significantly less efficient at thosefrequencies, and only lower power lasers are currently commerciallyavailable. Since lasers at the preferred frequencies, with the desiredparameters, are, not currently available, a laser suitable for this usehas to be designed from the ground up. The resonator and gain mediumhave to be designed. A laser with the selected frequency and a radiancevalue sufficient to facilitate a roughly collimated or nearly collimatedbeam must be constructed. To achieve good collimation of the beam, aradiance of at least 8 kW/m²/Steradian is needed, and even 800kW/m²/Steradian may be needed for higher power systems for efficientpower transmission. For small systems working at long distances, muchhigher radiance (up to 10 GW/m²/Steradian) may be designed in thefuture, according to similar principles. Receivers for use with radianceof less than that level need to be too large, which would make thesystem cumbersome.

Different mirror setups for the resonator have been used, specificallygood quality metallic mirrors made of Gold, Silver or Aluminium. Theseare found to reduce the lasing efficiency significantly. Much betterresults are achieved with dielectric material mirrors. Alternatively,Fresnel mirrors have one advantage in that they are low cost. Othermirrors that may be used are Bragg mirrors (which may be dielectric).The mirrors need to be positioned so as to form a stable, or a nearlystable resonator, or a resonator where photons are confined in space bya barrier inside the laser (such as in a fiber or diode laser) and again medium should be placed in the resonator between the mirrors in aposition allowing the gain medium to amplify the beam resonating insidethe resonator, such that it has a radiance of at least 8kW/m²/Steradian.

If the gain medium is capable of lasing at more than one wavelength, thedielectric mirrors can be selected to limit that wavelength to aspecific value. Alternatively, a filter can be used to fix the lasingfrequency.

Specifically, it is better if the mirrors have high reflectivity for atleast one wavelength between the first overtone of the C—H absorption at6940 cm⁻¹ and the second overtone of the C—H absorption at 8130 cm⁻¹.

Three different approaches may be used for the gain medium.

1. DPSS Design

In the DPSS design, the gain medium may be a Nd-doped YAG crystal,though YVO₄ crystals, GGG crystals and Glasses are also options for aclear host. Neodymium is most suitable for operation between the firstovertone of the C—H band and the second overtone of the C—H band sinceNd has a transition near ˜7450 cm⁻¹. The Nd ions need to be excited byabsorbing radiation, typically from 808 nm laser diodes, although otherwavelengths may be used. A Nd-based gain medium tends to lase at a muchhigher frequency unless a filter blocking the transition around 9400cm⁻¹ is added inside the resonator, or unless the unwanted radiationfrom the resonator is otherwise extracted. When such a filter is added,lasing commences at 7440-7480 cm⁻¹. Such filter action can be achievedusing a prism or a grating, instead of a filter or by proper chromaticdesign of the laser resonator.

2. Semiconductor Laser

As an alternative, a semiconductor-based design may be proposed. It ispossible to tune the wavelength of semiconductor lasers by altering thelasing bandgap of the semiconductor used. Semiconductors, especiallythose of the III-V type and more especially, though not exclusively,quantum dot types, having bandgaps of the order of 1 eV, emit light atthe desired frequencies between 6900 cm⁻¹ and 8200 cm⁻¹. Specificallybandgaps between 0.8 eV and 1.1 eV yield good results and are absorbed,at least partially, by essentially all commonly used polymers.

3. Various alternative designs may also be used in the systems describedin this disclosure, such as Nd doped fiber lasers, that may includeBragg mirrors and/or fiber loop mirrors. Alternatively Raman shiftedfiber lasers may also be used.

During operation the gain medium heats up, and should be cooled toprevent wavelength shift and efficiency degradation. If the gain mediumis properly cooled, then it is possible to increase the pump power orcurrent until a beam having radiance of at least 8 kW/m²/Steradian isemitted, having a frequency between 6900 cm⁻¹ and 8200 cm⁻¹. Such a beamcan be nearly collimated and will be attenuated by most organicmaterials, including polymers, allowing detection. However, it will notbe strongly absorbed by contaminations such as fingerprints.

The laser gain medium is typically configured to work at a temperaturebelow 150 degrees centigrade. If its temperature exceeds a level,typically around 250 centigrade, a number of problems may arise:.

Firstly, the efficiency of light emission may drop significantly, due topopulation of lower level excited states, especially in 3- and 4-levellasers, and also due to thermal recombination of charge carriers insemiconductors.

Secondly, the soldering of the gain medium, if such a thermal attachmentmethod is used, may be damaged.

Thirdly, thermal aberrations may occur which may cause beam degradation

Fourthly, the thermal expansion of the laser gain medium may bedifferent from that of its surroundings, which may cause mechanicalstress or even warping and fracture of the gain medium.

For those reasons, inter alia, the gain medium has to be thermallyattached to a cooling system. Typically the gain medium emits between0.1 and 100 W of heat from a surface that is between 1 mm² and 40 mm².In order for the temperature of the gain medium to be maintained at lessthan 150 degrees, the cooling system of the gain medium needs to have athermal resistance of less than 200 Kelvin per Watt, and for systemstransmitting higher powers, typically arising from more than 10 W ofelectrical power input, the thermal resistance should be significantlylower, and in many cases thermal resistance lower than 0.05 Kelvin/Wattis needed.

The surface of the cooling system is attached to the gain medium,typically using a third material such as solder or adhesive, which musthave an expansion coefficient that is compatible to both the expansioncoefficient of the gain medium itself and to the expansion coefficientof the front surface of the cooling system.

Typically such cooling systems may be either a passive heat sink, a heatsink with a fan, a Peltier element connected to a heat sink with orwithout a fan, or a liquid cooled cooling system. Alternatively, use maybe made of a stand-alone liquid circulating cooling system with activecirculation based on a circulating pump, or with passive circulation,based on heat pipes.

If the cooling system consists of a heat sink with a fan, its thermalresistance should be less than 0.1° Kelvin per Watt

If the cooling system is a passive heat sink, its thermal resistanceshould be less than 0.3° Kelvin per Watt

If the cooling system is a Peltier element, it needs to generate atleast 5 degrees temperature difference ΔT.

If the cooling system is an active liquid cooled cooling system, itshould be able to cover the entire span of thermal resistances mentionedhere.

A passive heat sink is preferred in systems designed for low cost andquiet operation, while a liquid cooled system is preferred for highpower systems. A heat sink with a fan or a fluid pump is used forsystems typically having more than 1 W electrical output and atransmitter having a small volume, such as less than approximately 1liter.

The gain medium is typically driven by a driver, supplying it withpower, which may be provided as electrical power as in the case of somesemiconductor gain media, or optical as in the case of othersemiconductor gain media or DPSS systems, or chemical or other forms ofenergy. The amount of power supplied by the driver determines the smallsignal gain achieved, which determines the working conditions andemission of the laser, while the saturated gain of the gain medium isgenerally a function of the material selected for the gain medium,though not always in a simple linear fashion, and ultimately, theradiance emitted from the laser. Such a laser driver might have two ormore operational states, one used for power transmission, and the othersused for other functions of the system, such as target seeking, setup,and information transmission. It is important that the laser driverproduces stable emission (with regards to power and beam parameters) inboth working conditions, although stable operation during powertransmission is more important.

To convert the optical beam into electricity again, so that useful poweris delivered, an optical-to-electric power converter, typically aphotovoltaic cell, should be used. As with the lasers, suitablephotovoltaic cells tailored to the frequency of the beam used, are notcommonly available as off-the-shelf components, and a custom cell isrequired. The bandgap of the photovoltaic semiconductor should beslightly smaller than the bandgap of the gain medium used, so that thebeam frequency is absorbed efficiently by the semiconductor. If not, theconversion efficiency will be very poor. On the other hand, if thebandgap used is too small, then a poor efficiency system is achieved.Also the conductors on the photovoltaic cell need to be tailored to theradiance of the beam used—the higher the radiance, the thicker theconductors needed.

Since the bandgap of the laser gain medium should be in the range0.8-1.1 eV, and the bandgap of the photovoltaic cell used must be lower,and since a single junction photovoltaic cell typically produces avoltage that is about 60-80% of the bandgap energy divided by theelectron charge, a single junction cell tailored to the laser frequencyyields a very low voltage, typically 0.3-0.8V, and typically a highcurrent, assuming an output power of a few watts, as required by apractical system. The conductors on the semiconductor need to be thickenough to carry the generated current without significant (e.g. >5%)losses. Typically the series resistance of the conductors needs to bebelow 1 Ohm, or even better, below 0.1 Ohm, and the heat generatedshould be efficiently extracted from the photovoltaic cell as itsefficiency generally decreases with temperature.

This combination of low voltage combined with high power cannot beeasily converted to the higher voltages required to charge portabledevices, typically 3.3 or 5V. Furthermore, some systems, such ascommunication systems, may require voltages such as −48V, 12V or 3.8V.The system needs to supply a stable voltage, and at higher levels thanthe output voltage expected from the photovoltaic cells. A typicalmethod to increase the voltage of photovoltaic cells is to connect themin series, such as is described in U.S. Pat. No. 3,370,986 to M. F.Amsterdam et al, for “Photovoltaic Series Array comprising P/N and N/PCells”, which shows a typical configuration for yielding a highervoltage, while utilizing almost the same amounts of semiconductor and noadditional components, and is therefore the typically chosen solution.

However this solution is not suitable for systems such as thosedescribed in the present application, in which a laser having a radianceas high as 8 kW/m²/Steradian is used, especially since such a lasertypically does not have a uniform shaped beam. Furthermore, its beamshape may be variable in time and the pointing accuracy may be less thanoptimally desired. In such a situation it is virtually impossible todesign a compact and efficient system that will illuminate all the cellsuniformly. If the photovoltaic cells connected in series are notuniformly illuminated, they do not produce the same current. In such acase the voltage will indeed be increased to the desired level but thecurrent would drop to the current generated by the cell producing theleast current, usually the cell least illuminated. In such a situationefficiency will be very poor. There is thus a need for an improvedalternative method to increase the voltage.

One method of increasing the voltage of a single cell may be by chargingcapacitors in parallel, and then discharging them in series. This methodyields good results for low currents, but when current is increasedbeyond a certain level, the switching time becomes a dominant factor,influencing efficiency, which degrades with increasing switching time.

If the energy is converted to AC using a fast, low resistance, switchingmechanism, that AC current can be amplified using coupled inductancesand then converted to AC again. The increased voltage AC can beconverted to DC using a diode bridge and an energy storage device, suchas a capacitor or a battery. Such systems have advantages when thevoltage needs to be increased beyond twenty times that of the photocellvoltage. Another advantage of such a system is that the switching can bedone from the transmitter using the laser, thus saving receiver cost andcomplexity. Such systems have disadvantages when the voltage needs to beincreased by a factor of less than 10 or when size and volumelimitations are critical to the application.

Reference is now made to FIG. 13A, which shows a method of voltageconversion that is efficient and simple. In the configuration of FIG.13A, a single inductor may be used with a low resistance switchingmechanism and an energy storage device to increase the voltage of thephotovoltaic cell. In FIG. 13A, the square on the left is thephotovoltaic cell, the switch S, is a low resistance switch, such as aMOSFET, JFET, BJT, IGBT or pHEMT, the inductance L is connected to theoutput of the photovoltaic cell and the capacitor C acts as an energystorage device.

The following description assumes for simplicity the use of componentswith zero resistance. Taking resistance losses into account complicatesthe calculations, and is explained in a later section of thisdisclosure. The switching mechanism cycles the inductor between twoprimary operating phases: charging phase and discharging phase. In thecharging phase the inductor is connected in parallel with thephotovoltaic cell, by the closing of switch S. During this phase theinductor is being charged with the energy converted by the photovoltaiccell. The inductor energy increase is given by:

ΔE _(L) _(_) _(CH) =Vpv*I _(L) *T _(CH),

where

Vpv is the output voltage of the photovoltaic cell,

I_(L) is the average inductor current and

T_(CH) is the duration of the charging phase.

In the discharging phase, the inductor is connected between thephotovoltaic cell and the load by the opening of switch S. During thisphase, the energy delivered from the inductor to the output energystorage device is given by the inductor energy decrease:

ΔE _(C)=Vo*I _(L) *T _(DIS), where

Vo is the voltage of the energy storage device, which is typically veryclose to the desired output voltage of the device, and can therefore beapproximated as the output voltage of the system.

I_(L) is the average inductor current and

T_(DIS) is the duration of the discharging phase.

The energy delivered from the photovoltaic cell to the inductor duringthat phase is given by

ΔE _(L) _(_) _(DIS) =Vpv*I _(L) *T _(DIS).

The change in the inductor energy during that phase is the differencebetween the incoming and outgoing energy:

ΔE _(L) _(_) _(DIS) =Vpv*I _(L) *T _(DIS)−Vo*I _(L) *T _(DIS).

In steady state operation, the energy of the inductor at the end of thecycle returns to the same value it was at the beginning of the cycleyielding

ΔE _(L) _(_) _(CH) =−ΔE _(L) _(_) _(DIS),

which after substitution yields:

Vo=Vpv*(1+T _(CH) /T _(DIS)).

The voltage at the energy storage device is thus defined by thephotovoltaic cell voltage and the ratio of the charging and dischargingphase durations.

In the present system, however, the parasitic characteristics and otheraspects of the components might have a significant impact on conversionoperation and efficiency and care should be taken into account inselecting and using the right components, in order to allow the systemto operate efficiently. These elements are now considered, one by one:

Inductor

1. The inductance of the inductor defines the rate of change of theinductor current due to applied voltage, which is given by dI/dt=V/L,where dI/dt is the rate of current change, V is the voltage appliedacross the inductor and L is the inductance. In the context of thecurrent system, V is determined by the gain medium in the transmitter.Selection of a different gain medium causes change in the photon energy,which mandates consequent changes in the photovoltaic bandgap, and hencea change in the photovoltaic voltage. This then calls for selection of adifferent inductor and/or switching frequency. The switching rate mustbe fast enough to allow the inductor current to respond to changes inthe incoming power from the transmitter through theoptical-to-electrical power converter, and slow enough to avoidhigh-magnitude current ripple which contributes to power loss, inputvoltage ripple and output voltage ripple. The optimal value of theinductor should yield ripple current which is between 20% and 40% of themaximum expected input current, but systems may be operable between 10%and 60%. Rigorous analysis of the circuit parameters shows that in orderto achieve this objective, the value L, of the inductor measured inHenries, must be within the limits:

$L < {\frac{1}{1.28*10^{- 40}*f}*{E_{gain}}^{2}*\frac{1 - \frac{E_{gain}}{5*10^{- 19}*V_{output}}}{P_{{laser}\_ {driver}}}}$$L > {\frac{1}{3*10^{- 38}*f}*{E_{gain}}^{2}*\frac{\left( {1 - \frac{E_{gain}}{4*10^{- 20}*V_{output}}} \right)}{P_{{laser}\_ {driver}}}}$

where

f is the switching frequency measured in Hz.,

E_(gain) is the bandgap of the gain medium, measured in Joules,

V_(output) is the output voltage from the voltage converter, measured inVolts, and

P_(laser) _(_) _(driver) is the power pumped by the laser driver intothe gain medium, measured in Watts.

In order to successfully integrate the inductor into a mobile client,the inductance should typically be smaller than 10 mH, as inductors thatare suitable for the current required by mobile client charging andhaving volume limitations suitable for a portable application aretypically well below this value. Also inductors having inductances toosmall, such as 10 nH, will require such a high switching frequency thatit will severely limit the availability of other components in thesystem such as the switch, and the switching loss caused by such a highfrequency may be higher than the amount of power delivered by thephotovoltaic cell.

2. The serial resistance of the inductor, R_(parasitic), should be aslow as possible to minimize the conduction power loss: Typically, avalue which yields less than 10% efficiency drop is chosen: the serialresistance of the inductor, measured in Ohms should be less than

$R_{parasitic} \leq {\frac{1}{2*10^{- 40}}*\frac{{E_{gain}}^{2}}{P_{{laser}\_ {driver}}}}$

where

E_(gain) is the bandgap of the gain medium, measured in Joules,

P_(laser driver) is the power pumped by the laser driver onto the gainmedium, measured in Watts.

3. In a typical system the inductor serial resistance would be less than10Ω. The saturation current of the inductor is usually chosen to behigher than the expected inductor peak current, given by:

I _(SAT) >I _(PEAK) =Im+Vpv*(1−Vpv/Vo)/(2*L*f).

For extracting more than 10 mW of power from a single junctionphotovoltaic cell, the saturation current must be higher than 10 mW/0.8v=12.5 mA.

4. For reliable operation the inductor shall be rated at a highercurrent than the expected maximum input current. For extracting morethan 10 mW of power from a single junction photovoltaic cell, theinductor rated current must be higher than 10 mW/0.8 v=12.5 mA.

Switching Mechanism

1. The switching mechanism is usually made of two or more devices. Thefirst device, a main switch, when conducting, sets the inductor into thecharging phase. The second device can be either a diode (as in FIG. 13A)or a switch whose function is to connect the inductor to the load oroutput energy storage device, during the discharging phase, and todisconnect it from the load during the charging phase.

2. The switching mechanism should have low switch node capacitance tominimize switching losses:

P _(SW2)=0.5*Csw*Vo² *f.

For extracting more than 50% of the laser power, the switch nodecapacitance should be less than

${Csw} \leq {\frac{P_{{laser}\mspace{14mu} {driver}}}{V_{o}^{2}*f}.}$

3. In a typical system switch node capacitance would be less than 100 nFand more than 10 pF.

4. The serial resistance of the main switch in the switch node, thatswitch being either that connecting the inductor to the ground or thatconnecting the optical-to-electrical power converter to the inductor,should be less than

$R \leq \frac{{E_{gain}}^{2}}{2*10^{- 40}*P_{{laser}\mspace{14mu} {driver}}}$

In a typical system the switch serial resistance would be less than 10Ω.

Energy Storage Device

1. The energy storage device can be either a capacitor or a battery orboth.

2. The energy storage device is required to maintain the output voltageduring the charging phase, when the inductor is disconnected from theoutput. The capacitance of the storage device is chosen based of theswitching frequency, laser power and the desired output ripple voltage:

C _(OUT) >P _(LASER DRIVER)/(f*Vo*ΔVo)

where ΔVo is the desired output ripple voltage.

3. The energy storage device can also supply power to the load duringtemporary interruption of the optical path. For uninterrupted powersupply, the energy storage device should be able to store at least theamount energy equal to minimal operational output power (P_(OUT MIN))multiplied by the interruption time interval (T_(INT)):

E _(OUT) _(_) _(MIN) ≥P _(OUT) _(_) _(MIN) *T _(INT).

If a capacitor is used as the energy storage device, the capacitanceshould be larger than:

C _(OUT)≥2*E _(OUT/) V _(OUT) ².

For uninterrupted operation at minimal operational output power largerthan 10 mW and interruption time interval longer than 100 ms the storedenergy has to be larger than 1 mJ and the capacitance larger than 80 μF(assuming V_(OUT)=5V).

In some cases, the capacitor may serve as the energy storage device forthe client application. In such cases, the client application may bedesigned without any secondary energy storage device (the conventionallyused battery installed in the mobile device), and the energy storagedevice of the presently described systems would have to store enoughenergy to supply the power needs of the client device until the nextcharging event. In such cases, super capacitors having a capacitance atleast 0.5 F, and even above 10 F, may be used. In other cases, where thepower requirement of the client device is low, or when it has anindependent energy storage device such as the battery internallyinstalled in the device, or if the device does not need to operate whenno power is supplied, the capacitor used would typically be well beyond1 F. If a rechargeable battery is used as the energy storage device,then, similar to the capacitor logic above, if the battery is used onlyas means of regulating the voltage, but not as the means for maintainingpower supply to the client device between charging events, then theenergy capacity of the battery may advantageously be up to 100 times theenergy supplied during 100 cycles of the switch (typically below 0.1Wh), this level being determined according to the volume budget and costeffectiveness of the battery. On the other hand, if the battery is alsoused to power the client device between charging events, its capacityshould be at least large enough to store the energy needed by the clientdevice between charging events—typically above 0.1 Wh in the case of acellular phone. Batteries also have a volume limitation depending on theproduct in which they are intended to be used. Thus, the battery of aproduct that has some volume V, if incorporated externally to thedevice, would typically be limited to up to times the volume of thedevice, i.e. 3V. As an example of this rule of thumb, a battery used topower a cellular phone of 100 cc. volume would typically be limited toless than 300 cc. in volume. Such a battery would typically have acapacity of below 300 Wh. because of the above mentioned limitation.

The circuit in FIG. 13A is not the only possible topology. FIG. 13Bshows a different design that can achieve similar performancecharacteristics. The components roles, constraints and expected valuesfor FIG. 13B are the same as those listed for the circuit in FIG. 13A.The primary difference is that the positive and negative terminals ofthe output voltage are reversed.

In some applications the energy storage device may be preferably locatedinside the device which is intended to use the power received. In otherapplications, specifically those applications where short term operationis anticipated, and which does not require a regulated voltage, theenergy storage device may even be eliminated.

Point of Regulation

The power output of the photovoltaic cell depends on the incomingoptical power and load applied to it. The optimal loading condition willyield the maximal output power from the photovoltaic cell, therefore,the control mechanism of the voltage converter must regulate the loadingpoint. The control mechanism can be either designed to maintain constantvoltage between the cell terminals, which is known to be maximum poweroperating point for most conditions, or it can track the maximum poweroperating point by measuring the cell output power and seeking theoptimal cell voltage under any operating condition. The first approachis simpler; the second is more power efficient.

The generated laser beam needs to be directed towards the receiver. Inorder to direct the beam towards the receiver, a beam steering apparatusshould be used. Some beam steering sub-systems that could be usedinclude a moving mirror, a moving lens, an electro-optical modulator, amagneto-optical modulator, a set of motors moving the whole transmittersystem in one or more directions, or any other suitable beam deflectiondevice.

The beam steering apparatus should be controlled by a controller, mostconveniently the same controller used to control the laser driver.

The beam steering apparatus is configured to direct the >8kW/m²/Steradian beam in any of a number of directions.

The damage threshold of the beam steering apparatus needs to be able towithstand the beam's radiance.

For example, if the beam is focused on a mirror using a focusingmechanism having a numerical aperture of 0.5, the mirror needs towithstand a power density of at least 6.7 kW/m² for a beam having 8kW/m²/steradian. If a beam having higher radiance is used the mirrorshould be chosen so that it would have a higher damage thresholdcorrespondingly.

FIG. 14 shows the power reflected per square meter by a mirror when abeam of 8 kW/m²/steradian is focused upon it as a function of numericalaperture.

If a higher radiance beam is used, then the power reflected by themirror increases correspondingly in a linear manner.

Since the beam may be far from uniform, “hotspots”, sometimes having 10×irradiance compared to the beam average, may be generated.

Hence, mirrors should have a damage threshold which is at least as largeand preferably at least 10× that shown in FIG. 14, scaled to the actualbeam irradiance and numerical aperture of the focusing mechanism on themirror.

There is typically an optical front surface in the receiver, positionednear the photovoltaic cell and between the photovoltaic cell and thetransmitter, through which the beam enters the receiver, and which isneeded in order to protect the typically delicate structure of thephotovoltaic cell, and in many cases, in order to match the exteriordesign of the device where the power receiver is integrated in. Thefront surface may have a coating protecting it from scratches, such asCorning Gorilla Glass®, or similar, or may be treated to make it betterwithstand scratches. It may be also be treated to reduce the levels ofcontaminants, such as fingerprints and dust which may settle on it, orto reduce their optical effect, or it may be coated with ananti-reflection coating to reduce the level of light reflected from it.The front surface of the photovoltaic cell may also be coated. In somecases the front surface would be part of the structure of thephotovoltaic cell itself or coated on the photovoltaic cell.

While in some situations, it may be possible to reduce the amount ofreflection from the surface to below the safety threshold, by choosing avery low reflection anti-reflective coating, should the coating becontaminated or covered by either a liquid spilled on it, or afingerprint, such anti-reflective coating would be ineffective inreducing the amount of reflection, and typically, 3-4% of the incidentlight will be reflected in an uncontrolled direction. If such areflection is reflected in a diverging manner, its power density wouldsoon drop to safe levels. However, should the reflection be focused, thepower density may increase to unsafe levels. For this reason, it isimportant that the ROC (radius of curvature) of such a surface, at anypoint on it, should not be less than a predetermined value. In general,the reflection from the surface is intended to be only a small part ofthe incident light, thereby reducing the danger of any significant beamreflections, regardless of what nature or of what form the surfacecurvature takes. The level of reflected light may be variable, sinceeven the ˜4% reflection from an untreated glass surface may beincreased, if a layer of extraneous contaminant material on the surfacegenerates increased reflectivity. However, that reflection is expectednot exceed 20%, and will generally be substantially less than the 4% ofuntreated glass, such as in the case of AR coated glass, wherereflectivities of 0.1% or even less are common. Therefore, the surfaceis described in this disclosure, and is thuswise claimed, as havingproperties which reflect a small part of the incident light, thisdescription being used to signify less than 20% of the incident light,and generally less than the 4% of untreated glass.

Reference is now made to FIGS. 15A to 15C, which illustrateschematically methods of avoiding the above-mentioned unsafereflections, even for the small part of the incident light which may bereflected from the surface. FIG. 15A shows a situation where the surfaceis a concave surface, FIG. 15B shows a situation where the surface is aconvex surface, and FIG. 15C shows the situation where the surface is adiffusive surface. In FIG. 15A, an incident beam 110, having at least 8kW/m²/Steradian radiance, is directed towards photovoltaic cell 112,passing through a front surface 111, which may be the front surface ofthe photovoltaic cell. The front surface 111 reflects some of beam 110creating a focused beam 113 with a focal point 114 some distance fromthe surface. In order to ensure that focal point 114 does not presentany danger to an eye or skin, or other objects, the Radius of Curvature(ROC) of the surface 111 must be such that the beam is focused with lownumerical aperture, as in FIG. 15A, or that it be defocused, as in FIG.15B, or that it be diffused, as in FIG. 15C. To achieve theselimitations, if the surface is concave looking from the transmittertowards the photovoltaic cell, as in FIG. 15A, its ROC must be largerthan 1 cm, and if higher power systems are used, typically above 0.5 Wof light, it should be greater than 5 cm. Alternatively, the surface ROCcan be negative, as in FIG. 15B, but the ROC cannot be in the range 0-1cm. These limitations will ensure that the reflected beam of light has afocal point which is either virtual, i.e. associated with a divergingreflected beam, or at least 1 cm in front of the surface, such that therisk generated by the focus is significantly reduced. The surface mayalso have numerous regions with smaller curvatures, creating a diffusivesurface, as in FIG. 15C, which significantly helps reducing the risk ofa dangerous focal point. In such a case, the radius of curvature of eachsub section of the surface may be smaller than 1 cm without creating afocal point. Furthermore, if the surface is split into multiple zones,each zone may have smaller curvature.

In order to operate safely, the system also needs to be able to directthe power beam to the photovoltaic cell so that it is blocked by it, andnot be directed at some unsafe region. In order to accomplish that, adetector should be positioned to provide indication of the impingementof the beam on the receiver. Such a detector should typically bepositioned in the receiver, but configurations where such a detector islocated in the transmitter are also possible, in which case the detectorshould be responsive to a phenomenon occurring due to the impact of thebeam on the receiver. Such a transmitter-associated system may includeimage acquisition and processing of optical information received fromthe receiver, such as the reflection of the beam from a barcode printedon the receiver, so that the transmitter may detect the barcode'sillumination pattern. Reflections from a retro reflector or retroreflectors or arrays or patterns thereof may be positioned on thereceiver and such reflection may be detected in the transmitter, eitherby way of image processing, by measuring back reflection or by measuringcoherence effects of the reflection. The detector may be a current orvoltage sensor positioned in the receiver, a photodiode in the receiveror in the transmitter, or an imaging device which may be either in thetransmitter or the receiver. A retro-reflector in the vicinity of thephotovoltaic cell may also be used, in combination with an additionaldetector in the transmitter, detecting light reflected from theretroreflector.

The detector, upon detecting the beam of light impinging on thephotovoltaic cell, sends a signal accordingly to the system controller.If the detector is in the receiver, such signalling may be donewirelessly, using a communication channel which may be RF, IR, visiblelight, UV, modulation of the beam, TCP/IP, or sound. The systemcontroller is usually located in the transmitter, but may also belocated in a main control unit, which may even be on a computer networkfrom the transmitter. On receipt of the signal, the controller respondsby performing at least one of:

(a) Changing the state of the laser driver.

(b) Changing the operational properties of the beam steering apparatus,such as the direction to which it directs the beam, or the speed inwhich such direction is changed.

Reference is now made to FIG. 16, which is a schematic diagram showing adetailed description of the complete system. The system comprisestransmitter 21 and receiver 22. In general, the transmitter and receiverwill be located remotely from each other, but are shown in FIG. 16, forconvenience, to be close to each other. Beam 15 transfers power fromtransmitter 21 to receiver 22.

On the receiver 22, the front surface 7 reflects a small part ofincident beam 15 as a reflected beam 16, while either diffusing it orcreating a virtual focal point behind front surface 7, or a real focalpoint at least 1 cm in front of surface 7. After transmission throughthe at least partially transparent surface 7, beam 15 impinges on theoptical-to-electrical power converter 1.

The optical-to-electrical power converter 1 may be enclosed in a packagethat may have a front window, which may be surface 7 or a separatewindow. It may also be coated to have an external surface adapted tofunction as an interface with the air, or the adhesive or the glasssurrounding it. In a typical configuration, the optical-to-electricalpower converter 1 could be a junction of semiconductor layers, whichtypically have conductors deposited on them. In many embodiments surface7 would be coated on, or be the external surface of one of thesesemiconductor layers.

Signalling detector 8 indicates that beam 15 is impinging onphotovoltaic cell 1 and transmits that information to the controller 13,in this example system, located in the transmitter 21. The controlsignal is transmitted by a link 23 to a detector 24 on the transmitter.

Electrical power converter 1, has a bandgap E8 and typically yields avoltage between 0.35 and 1.1V, though the use of multi-junctionphotovoltaic cells may yield higher voltages. Power flows from thephotovoltaic cell 1 through conductors 2 a and 2 b, which have lowresistance, into inductor 3 which stores some of the energy flowingthrough it in a magnetic field.

Automatic switch 4, typically a MOSFET transistor connected to a controlcircuit (not shown in FIG. 16), switches between alternating states,allowing the current to flow through the inductor 3 to the ground for afirst portion of the time, and for a second portion of the time,allowing the inductor to emit its stored magnetic energy as a current ata higher voltage than that of the photovoltaic cell, through diode 5 andinto load 6, which can then use the power.

Automatic switch 4 may be operating at a fixed frequency or at variablefrequency and/or duty cycle and/or wave shape which may either becontrolled from the transmitter, or be controlled from the client load,or be based on the current, voltage, or temperature at the load, or bebased on the current, voltage or temperature at automatic switch 4, orbe based on the current, voltage or temperature emitted by theoptical-to-electrical power converter 1, or be based on some otherindicator as to the state of the system.

The receiver may be connected to the load 6 directly, as shown in FIG.16, or the load 6 can be external to the receiver, or it may even be aseparate device such as a cellphone or other power consuming device, andit may be connected using a socket such as USB/Micro USB/Lightning/USBtype C.

In most cases there would also be an energy storage device, such as acapacitor or a battery connected in parallel to load 6, or load 6 mayinclude an energy storage device such as a capacitor or a battery.

Transmitter 21 generates and directs beam 15 to the receiver 22. In afirst mode of operation, transmitter 21 seeks the presence of receivers22 either using a scanning beam, or by detecting the receiver usingcommunication means, such as RF, Light, IR light, UV light, or sound, orby using a camera to detect a visual indicator of the receivers, such asa retro-reflector, or retro-reflective structure, bar-code, highcontrast pattern or other visual indicator. When a coarse location isfound, the beam 15, typically at low power, scans the approximate areaaround receiver 22. During such a scan, the beam 15 impinges onphotovoltaic cell 1. When beam 15 impinges on photovoltaic cell 1,detector 8 detects it and signals controller 13 accordingly.

Controller 13 responds to such a signal by either or both of instructinglaser driver 12 to change the power P it inputs into gain medium 11 andor instructing mirror 14 to alter either its scan speed or direction itdirects the beam to or to hold its position, changing the scan stepspeed. When gain medium 11 receives a different power P from the laserpower supply 12, its small signal gain—the gain a single photonexperiences when it transverses the gain medium, and no other photonstraverse the gain medium at the same time,—changes. When a photon,directed in a direction between back mirror 10 and output coupler 9passes through gain medium 11, more photons are emitted in the samedirection—that of beam 15—and generate optical resonance between backmirror 10 and output coupler 9.

Output coupler 9 is a partially transmitting mirror, having reflectanceR, operating at least on part of the spectrum between the first overtoneof the C—H absorption at 6940 cm⁻¹ and the second overtone of the C—Habsorption at 8130 cm⁻¹, and is typically a multilayer dielectric orsemiconductor coating, in which alternating layers of differentrefractive index materials are deposited on a substrate, which istypically glass, plastic or the surface of gain medium 11. AlternativelyFresnel reflection can be used if the gain medium is capable ofproviding sufficient small signal gain or has a high enough refractiveindex, or regular metallic mirrors can be used. A Bragg reflector mayalso be used, should the gain medium be either a semiconductor or afiber amplifier. Output coupler 9 may also be composed of a highreflectance mirror combined with a beam extractor, such as asemi-transparent optical component that transmits a part of the lightand extracts another part of the light from the forward traveling waveinside the resonator, but typically also a third portion extracted fromthe backwards propagating wave inside the resonator.

Back reflector 10 should be a high reflectance mirror, although a smallamount of light may back-leak from it and may be used for monitoring orother purposes, working at least on part of the spectrum between thefirst overtone of the C—H absorption at 6940 cm⁻¹ and the secondovertone of the C—H absorption at 8130 cm⁻¹. It may typically beconstructed of alternating layers of different refractive indexmaterials deposited on a substrate, usually glass, metal or plastic.Alternatively Fresnel reflection can be used if the gain medium iscapable of providing sufficient small signal gain, or regular metallicmirrors can be used. A Bragg reflector may also be used should the gainmedium be either a semiconductor or a fiber amplifier.

Gain medium 11 amplifies radiation between the first overtone of the C—Habsorption at 6940 cm⁻¹ and the second overtone of the C—H absorption at8130 cm⁻¹, although not necessarily over the whole of this spectralrange. It is capable of delivering small signal gain larger than theloss caused by output coupler 9 when pumped with power P by laser driver12. Its area, field of view, and damage thresholds should be largeenough to maintain a beam of at least 8 kW/m²/Steradian/(1-R), where Ris the reflectance of output coupler 9. It may be constructed of eithera semiconductor material having a bandgap between 0.8-1.1 eV or of atransparent host material doped with Nd ions, or of another structurecapable of stimulated emission in that spectral range. Gain medium 11 ispositioned in the optical line of sight from the back reflector 10 tooutput coupler 9, thus allowing radiation reflected by the backreflector 10 to resonate between the back reflector 10 and the outputcoupler 9 through gain medium 11.

For the exemplary implementation where the gain medium 11 is asemiconductor having a bandgap between 0.8-1.1 eV, it should bepreferably attached to a heat extracting device, and may be pumpedeither electrically or optically by laser driver 12.

In the exemplary implementation where the gain medium 11 is atransparent host, such as YAG, YVO₄, GGG, or glass or ceramics, dopedwith Nd ions, then gain medium 11 should preferably also be in opticalcommunication with a filter for extracting radiation around 9400 cm⁻¹from that resonating between back mirror 10 and output coupler 9.

The beam steering apparatus 14 is shown controlled by controller 13. Itcan deflect beam 15 into a plurality of directions. Its area should belarge enough so that it would contain essentially most of beam 15 evenwhen tilted to its maximal operational tilt angle. Taking a simplistic2D example, if beam 15 were to be a collimated 5 mm diameter (1/e²diameter) Gaussian beam, and the beam steering apparatus were to be asingle round gimballed mirror centered on the beam center, and if themaximal tilt required of the mirror is 30 degrees, and assuming thatbeam steering apparatus 14 has no other apertures, then if the mirrorhas a 5 mm diameter like that of the beam, it would have anapproximately 13% loss at normal incidence to the beam, butapproximately 60% loss at 60 degrees tilt angle. This would severelydamage the system's performance. This power loss is illustrated in thegraph of FIG. 17.

At the beginning of operation, controller 13 commands laser driver 12and mirror 14 to perform a seek operation. This may be done by aimingbeam 15, with the laser driver 12 operating in a first state, towardsthe general directions where a receiver 22 is likely to be found. Forexample, in the case of a transmitter mounted in a ceiling corner of aroom, the scan would be performed downwards and between the two adjacentwalls of the room. Should beam 15 hit a receiver 22 containing anoptical-to-electric power converter 1, then detector 8 would signal assuch to controller 13. So long as no such signal is received, controller13 commands beam steering 14 to continue directing beam 15 in otherdirections, searching for a receiver. If such a signal is received fromdetector 8, then controller 13 may command beam steering 14 to stop orslow down its scan to lock onto the receiver, and to instruct laserdriver 12 to increase its power emission. Alternatively controller 13may note the position of receiver 22 and return to it at a later stage.

When laser driver 12 increases its power emission, the small signal gainof gain medium 11 increases, and as a result beam 15 carries more powerand power transmission begins. Should detector 8 detect a power lossgreater than a threshold, which may be pre-determined or dynamicallyset, and which is typically at a level representing a significantportion of the maximal permissible exposure level, and which is alsotypically greater than the system noise figure, these conditionsimplying either that beam 15 is no longer aimed correctly at theoptical-to-electrical power converter 1, or that some object has enteredthe beam's path, or that a malfunction has happened, controller 13should normally command laser driver 12 to change its state, by reducingpower to maintain the required safety level. If another indication ofsafe operation is present, such as an indication from the user as to thesafety of transmission, which may be indicated by a user interface or anAPI, or an indication of safe operation from a second safety system, thecontroller may command the laser to increase power to compensate for thepower loss. The controller 13 may also command the beam steeringassembly 14 to perform a seeking operation again.

There may be two different stages in the seek operation. Firstly, acoarse search can be performed using a camera, which may search forvisual patterns, for a retro reflector, for high contrast images, for aresponse signal from receivers or for other indicators, or by using thescanning feature of beam steering 14. A list of potential positionswhere receivers may be found can thus be generated. The second stage isa fine seek, in which the beam steering mirror 14 directs beam 15 in asmaller area until detector 8 signals that beam 15 is impinging on anoptical-to-electrical power converter 1.

Reference is now made to FIG. 18, shows an example cooling system forthe gain medium 11 of the system of FIG. 16. Although the reflectors 9,10 are shown as separate optical elements, it is to be understood thatone or both of them may be coated directly on the gain medium end facesfor simplifying the system. Gain medium 11 converts the power itreceives from the laser driver 12 into both heat and photons, and wouldtypically degrade the system performance if the gain medium were to beheated above a certain temperature. For that reason, gain medium 11 isattached to heatsink 34 using a bonding agent 33 which is preferably aheat conducting solder having low thermal resistance. Bonding agent 33may also be a conductive adhesive. Bonding agent 33 may have a thermalexpansion coefficient which is between that of gain medium 11 and heatsink 34. Heat sink 34 may typically be a low thermal resistance heatsinkmade out of metal, which may be equipped with fins for increasing itssurface area or an external fluid pumping system such as a fan or aliquid pump 35.

Reference is now made to FIG. 19, which is a schematic diagram showing adetailed description of the system of FIG. 16, but further incorporatinga safety system 31, constructed and operable according to the methodsand systems described in the present application. Although shown in FIG.19 as a separate module, in order to show the additional inputs providedthereto, the safety system can be incorporated into the controller 13,and is generally describe3d and may be claimed thereas. As describedabove, the system comprises transmitter 21 and receiver 22. In general,the transmitter and receiver will be located remotely from each other,but are shown in FIG. 19, for convenience, to be close to each other.Beam 15 transfers power from transmitter 21 to receiver 22.

On the receiver 22, the front surface 7 reflects a small part ofincident beam 15 as a reflected beam 16, while either diffusing it orcreating a virtual focal point behind front surface 7, or a real focalpoint at least 1 cm in front of surface 7. After transmission throughthe at least partially transparent surface 7, beam 15 impinges on theoptical-to-electrical power converter 1 having a semiconductor layerhaving thickness T and an absorption coefficient to said optical beam15. The thickness of the layer is dependent on the designed wavelengthof the beam, and, when measured in cm, should be greater than 0.02 timesthe reciprocal of the absorption coefficient of the optical beam in thesemiconductor layer, as described in further details on FIG. 20 below.

The optical-to-electrical power converter 1 may be enclosed in a packagethat may have a front window, which may be surface 7 or a separatewindow. It may also be coated to have an external surface adapted tofunction as an interface with the air, or the adhesive or the glasssurrounding it. In a typical configuration, the optical-to-electricalpower converter 1 could be a junction of semiconductor layers, whichtypically have conductors deposited on them. In many embodiments surface7 would be coated on, or be the external surface of one of thesesemiconductor layers.

Signaling detector 8 indicates that beam 15 is impinging on photovoltaiccell 1 and transmits that information to the controller 13, in manycases it also transmits other data such as the power received, theoptical power received, identification information, temperatures of thereceiver and photovoltaic as well as information relayed from the clientdevice, which may be control information. In this example, systemcontroller 13 is located in the transmitter 21, but may also be locatedremotely therefrom. The control signal is transmitted by a link 23 to adetector 24 on the transmitter.

Safety system 31 receives information from various sources, furtherdetailed in FIG. 21 below, and especially may receive information from asmall portion of the beam 15 coupled out by beam coupler 32, and fromthe signaling detector 8, usually through a data channel between thepower receiver and the power transmitter. Safety system 31 outputssafety indications to control unit 13.

Electrical power converter 1, has a bandgap E8 and typically yields avoltage between 0.35 and 1.1V, though the use of multi-junctionphotovoltaic cells may yield higher voltages. Power flows from thephotovoltaic cell 1 through conductors 2 a and 2 b, which have lowresistance, into inductor 3, which stores some of the energy flowingthrough it in a magnetic field.

Automatic switch 4, typically a MOSFET transistor connected to a controlcircuit (not shown in FIG. 19), switches between alternating states,allowing the current to flow through the inductor 3 to the ground for afirst portion of the time, and for a second portion of the time,allowing the inductor to release its stored magnetic energy as a currentat a higher voltage than that of the photovoltaic cell, through diode 5and into load 6, which can then use the power.

Automatic switch 4 may operate at a fixed frequency or at variablefrequency and/or duty cycle and/or wave shape, which may be controlledeither from the transmitter, or be controlled from the client load, orbe based on the current, voltage, or temperature at the load, or bebased on the current, voltage or temperature at automatic switch 4, orbe based on the current, voltage or temperature emitted by theoptical-to-electrical power converter 1, or be based on some otherindicator as to the state of the system.

The receiver may be connected to the load 6 directly, as shown in FIG.16, or the load 6 can be external to the receiver, or it may even be aseparate device such as a cellphone or other power consuming device, andit may be connected using a socket such as USB/Micro USB/Lightning/USBtype C. The receiver typically further comprises a load ballast used todissipate excess energy from the receiver, which may not be needed bythe client.

In most cases, there would also be an energy storage device, such as acapacitor or a battery connected in parallel to load 6, or load 6 mayinclude an energy storage device such as a capacitor or a battery.

Transmitter 21 generates and directs beam 15 to the receiver 22. In afirst mode of operation, transmitter 21 seeks the presence of receivers22 either using a scanning beam, or by detecting the receiver usingcommunication means, such as RF, Light, IR light, UV light, or sound, orby using a camera to detect a visual indicator of the receivers, such asa retro-reflector, or retro-reflective structure, bar-code, highcontrast pattern or other visual indicator. When a coarse location isfound, the beam 15, typically at low power, scans the approximate areaaround receiver 22. During such a scan, the beam 15 should impinge onphotovoltaic cell 1. When beam 15 impinges on photovoltaic cell 1,detector 8 detects it and signals controller 13 accordingly.

Controller 13 responds to such a signal by either or both of instructinglaser driver 12 to change the power P input into gain medium 11 and orinstructing mirror 14 to alter either its scan speed or direction itdirects the beam to or to hold its position, changing the scan stepspeed. When gain medium 11 receives a different power P from the laserpower supply 12, its small signal gain—the gain a single photonexperiences when it transverses the gain medium, and no other photonstraverse the gain medium at the same time—changes. When a photon,directed in a direction between back mirror 10 and output coupler 9passes through gain medium 11, more photons are emitted in the samedirection—that of beam 15—and generate optical resonance between backmirror 10 and output coupler 9.

Output coupler 9 is a partially transmitting mirror, having reflectanceR, operating at least on part of the spectrum between the first overtoneof the C—H absorption at 6940 cm-1 and the second overtone of the C—Habsorption at 8130 cm-1, and is typically a multilayer dielectric orsemiconductor coating, in which alternating layers of differentrefractive index materials are deposited on a substrate, which istypically glass, plastic or the surface of gain medium 11.Alternatively, Fresnel reflection can be used if the gain medium iscapable of providing sufficient small signal gain or has a high enoughrefractive index, or regular metallic mirrors can be used. A Braggreflector may also be used, should the gain medium be either asemiconductor or a fibre amplifier. Output coupler 9 may also becomposed of a high reflectance mirror combined with a beam extractor,such as a semi-transparent optical component that transmits a part ofthe light and extracts another part of the light from the forwardtraveling wave inside the resonator, but typically also a third portionextracted from the backwards propagating wave inside the resonator.

Back reflector 10 should be a high reflectance mirror, although a smallamount of light may be allowed to back-leak from it and may be used formonitoring or other purposes. These optical characteristics shouldoperate at least on part of the spectrum between the first overtone ofthe C—H absorption at 6940 cm-1 and the second overtone of the C—Habsorption at 8130 cm-1. It may typically be constructed of alternatinglayers of different refractive index materials deposited on a substrate,usually glass, metal or plastic. Alternatively, Fresnel reflection canbe used if the gain medium is capable of providing sufficient smallsignal gain, or regular metallic mirrors can be used. A Bragg reflectormay also be used should the gain medium be either a semiconductor or afiber amplifier.

Gain medium 11 amplifies radiation between the first overtone of the C—Habsorption at 6940 cm-1 and the second overtone of the C—H absorption at8130 cm-1, although not necessarily over the whole of this spectralrange. It is capable of delivering small signal gain larger than theloss caused by output coupler 9 when pumped with power P by laser driver12. Its area, field of view, and damage thresholds should be largeenough to maintain a beam of at least 8 kW/m2/Steradian/(1-R), where Ris the reflectance of output coupler 9. It may be constructed of eithera semiconductor material having a bandgap between 0.8-1.1 eV or of atransparent host material doped with Nd ions, or of another structurecapable of stimulated emission in that spectral range. Gain medium 11 ispositioned in the optical line of sight from the back reflector 10 tooutput coupler 9, thus allowing radiation reflected by the backreflector 10 to resonate between the back reflector 10 and the outputcoupler 9 through gain medium 11.

For the exemplary implementation where the gain medium 11 is asemiconductor having a bandgap between 0.8-1.1 eV, it should preferablybe attached to a heat extracting device, and may be pumped eitherelectrically or optically by laser driver 12.

In the exemplary implementation where the gain medium 11 is atransparent host, such as YAG, YVO4, GGG, or glass or ceramics, dopedwith Nd ions, then gain medium 11 should preferably also be in opticalcommunication with a filter for extracting radiation around 9400 cm-1from the radiation resonating between back mirror 10 and output coupler9.

The beam steering apparatus 14 is shown controlled by controller 13. Itcan deflect beam 15 into a plurality of directions. Its area should belarge enough so that it would contain essentially most of beam 15 evenwhen tilted to its maximal operational tilt angle. Taking a simplistic2D example, if beam 15 were to be a collimated 5 mm diameter (1/e2diameter) Gaussian beam, and the beam steering apparatus were to be asingle round gimballed mirror centered on the beam center, and if themaximal tilt required of the mirror is 30 degrees, and assuming thatbeam steering apparatus 14 has no other apertures, then if the mirrorhas a 5 mm diameter similar to that of the beam, it would have anapproximately 13% loss at normal incidence to the beam, butapproximately 60% loss at 60 degrees tilt angle. This would severelydamage the system's performance. This power loss is illustrated in thegraph of FIG. 17, and in FIGS. 22 and 23 below.

At the beginning of operation, controller 13 commands laser driver 12and mirror 14 to perform a seek operation. This may be done by aimingbeam 15, with the laser driver 12 operating in a first state, towardsthe general directions where a receiver 22 is likely to be found. Forexample, in the case of a transmitter mounted in a ceiling corner of aroom, the scan would be performed downwards and between the two adjacentwalls of the room. Should beam 15 hit a receiver 22 containing anoptical-to-electric power converter 1, then detector 8 would signal assuch to controller 13. So long as no such signal is received, controller13 commands beam steering 14 to continue directing beam 15 in otherdirections, searching for a receiver. If such a signal is received fromdetector 8, then controller 13 may command beam steering 14 to stop orslow down its scan to lock onto the receiver. Controller 13 then waitsfor safety system 31 to generate a signal indicating that it is safe tooperate, and once such a safety signal is received from safety system31, controller 13 may instruct laser driver 12 to increase its poweremission. Alternatively, controller 13 may note the position of receiver22 and return to it at a later stage, which may be done even without thepresence of a safety signal.

When laser driver 12 increases its power emission, the small signal gainof gain medium 11 increases, and as a result beam 15 carries more powerand power transmission begins. Should detector 8 detect a power lossgreater than a certain threshold, safety system 31, may report such asituation to controller 13, which should normally command laser driver12 to change its state, by reducing power to maintain the requiredsafety level. Such a power loss threshold may be pre-determined ordynamically set, and is typically at a level representing a significantportion of the maximal permissible exposure level, and is also typicallygreater than the system noise figure. Such conditions imply either thatbeam 15 is no longer aimed correctly at the optical-to-electrical powerconverter 1, or that some object has entered the beam's path, or that amalfunction has happened. If another indication of safe operation ispresent, such as an indication from the user as to the safety oftransmission, which may be indicated by a user interface or an API, oran indication of safe operation from a second safety system, thecontroller may command the laser to increase power to compensate for thepower loss. The controller 13 may also command the beam steeringassembly 14 to perform a seek operation again.

There may be two different stages in the seek operation. Firstly, acoarse search can be performed using a camera, which may search forvisual patterns, for a retro reflector, for high contrast images, for aresponse signal from receivers (such as a blinking light from a LED orother light source), or for other indicators, or the coarse search maybe performed by using the scanning feature of beam steering unit 14. Alist of potential positions where receivers may be found can thus begenerated. The second stage is a fine seek, in which the beam steeringmirror 14 directs beam 15 in a smaller area until detector 8 signalsthat beam 15 is impinging on an optical-to-electrical power converter 1.

Reference is now made to FIG. 20, which is a schematic view of theoptical to electrical power converter, marked as item 1 in FIGS. 16, 19.Beam 15 impinges on photovoltaic cell 106, which is thermally connectedto heat removal system 107. Beam 15 is absorbed by absorbing layer 108causing a current to flow in conductors 111, the current being normallycollected by a bus. The optical power absorbed by absorbing layer 108 istypically converted into electrical power and into heat. The electricalpower is transferred through conductors 111 and a bottom electrode,while the thermal energy is evacuated mostly through a cooling system107. Conductors 111 cast a shadow on absorbing layer 108 decreasing itsefficiency, and it should therefore be made of a high conductivitymaterial, such as materials having less than 3E-6 Ohm*Meter specificelectrical resistance. It can be shown that such conductors should havea thickness in meters that is at least

${\frac{0.034*P\; \rho}{V^{2}*\chi}\mspace{14mu} m},$

where P is the power absorbed by the photovoltaic, measured in watts

ρ is the specific electrical resistance of the conductors,

V is the voltage emitted by the photovoltaic cell at its maximal powerpoint, and

χ is the fraction of the area of the absorbing layer covered byconductors

The absorbing layer also needs to be thick enough to absorb most of beam15 impinging on it. In order to do so, the thickness of absorbing layer108 measured in meters, needs to be at least 0.02/μ₁₀, where μ₁₀ is thedecadic attenuation coefficient measured in 1/m.

Reference is now made to FIG. 21, which shows a block diagram view ofthe safety system 31 of FIG. 19. Safety system 31 receives inputs fromvarious sensors and sub-systems and sends output to controller 13, inthose situations where the safety system is not an integral part of thecontroller 13, or when parts of the safety system are in an externalcontrol unit. Safety system 13 can also sometimes receive inputs fromthose various sensors and sub-systems. Such inputs can be fromwavelength sensor 407, which monitors primarily the beams wavelength, inorder to provide information needed for estimating the safety limitsassociated with the beam. It may also receive information from a beamanalyzer (401) which may monitor the beam's properties such as shape,M², symmetry, polarization, power, divergence, coherence and otherinformation related to the beam and to the above parameters. It usuallyalso receives information measured by external sub-systems through RFlink 402. Temperature measurement of various components in thetransmitter, receiver and surrounding area can be provided bytemperature sensor(s) 403. It may receive an image from camera 404,which may be visible, thermal, IR or UV, and from power meter 406measuring the beam's power at various positions. In many cases, theprimary sensors connected to safety system 31 may be intrusion sensors(405) which monitor the beam for foreign objects traversing orapproaching the beam path or its surroundings. It may also receiveinputs from other sensors such as current, voltage, smoke, humidity andother environmental sensors. Upon reception of those inputs, or at aprescheduled time, safety system 31 assesses the potential for asecurity breach and issues a notification to controller 13 if thatassessment exceeds a predetermined threshold.

Reference is now made to FIG. 22 showing a beam deflected by a mirrorrotating on a gimbaled axis, or on gimbaled axes. Beam 15 impinges onmirror 332 rotating around 2 axes in two dimensions. Beam 15 forms aspot 333 on mirror 332 and is deflected in a different direction. Theimportance of selecting the proper center of rotation and mirrordimensions becomes clearer by referring to FIG. 23. In FIG. 23, themirror 332 has now rotated so that beam 15 is now deflected at a largerangle compared to FIG. 22. Due to the increased angle, spot 333 nowforms a projection on the mirror surface longer than the effectivelength of mirror 332, so that a significant portion of beam 15, thatportion being marked 333A, is now spilled around mirror 332. Thisspilling reduces the brightness of beam 15, both by reducing its powerand by cutting off its edges, which in most cases, degrades the beamquality in the far field. Typically the beam diameter is reduced in thenear field close to the mirror, or on images of the near field, andincreased in the far field. In order to achieve a minimally dimensionedsystem, working at relatively high efficiency, it is important tomaintain the brightness as high as possible. This can be done byreducing the brightness loss experienced by beam 15, across all angleswithin the field of view of the system. This may be done by mounting themirror so that its center of rotation is essentially close to the beam'scenter, measured either by a weighted average of the beam's intensity,or by a cross section of the beam's diameter at a certain intensity, orby the center of an elliptic aperture through which the beam passes. Itis noted that, in contrast to the length projection, the width of thebeam projection on the mirror is unchanged with impingement angle.

FIG. 24 shows a schematic representation of an intensity profile of atypical beam, contour 1 marks the 90% line of the maximum intensity,contour 2 marks the 80% of the maximum intensity line, contour 3 theFWHM (Full Width at Half Maximum) intensity line, contour 4 the 1/eintensity line, contour 5 the 1/e² intensity line, and contour 6 the1/e⁴ intensity line. Point 231 is approximately at the weighted averagepoint of the beam, point 232 is at the center of the first contour andpoint 233 is at the center of the 6^(th) contour, all being valid pointsat which to place the center of rotation of the mirror. However, placingthe center of rotation beyond such points will require a larger mirrorin order to maintain high radiance efficiency of the gimballed mirror.

Maintaining high radiance efficiency for other components is also ofimportance, although the gimballed mirror and the first lens followingthe laser are typically the limiting components for the radianceefficiency.

FIG. 25 shows a schematic side view of a laser diode from a directionperpendicular to the fast axis of the laser, also showing a lens 242 formanipulating, and usually nearly collimating the fast axis. In mostcases lens, 242 is a compound lens, comprising several optical elements.Laser 241 is connected to heat sink 243 and emits beam 15, intointerface layer 244, which has a refractive index n for the wavelengthassociated with beam 15. The value of n is 1.000293 in the case of anair interface at 532 nm, and higher in the case of oil or opticalcement. Beam 15 has a divergence in at least one direction. The FWHMcontour of beam 15 at the front surface of lens 242 has a diameter d,defined as the maximal distance between any two points on the FWHMcontour. In order to have high radiance efficiency, lens 242 should havea numerical aperture NA with respect to the emitter of laser 241, of atleast:

${NA} > \frac{0.36*d}{{BFL}\sqrt{1 + \left( \frac{d}{2*{BFL}*n} \right)^{2}}}$

where

d is the FWHM diameter measured in mm between the two furthest points onthe beam's

FWHM contour on the lens front surface,

BFL is the back focal length of the lens measured in mm, and

n is the refractive index of the interface layer between the laser andthe lens.

If a lens having a smaller Numerical Aperture is used, the radiance ofthe beam is reduced by the lens resulting in either poor efficiency ofthe system, or in larger receiver, which may be disadvantageous in manysituations. Using a smaller NA will also result in heating of the lensholder, which may cause two harmful effects—firstly it may thermallyexpand and move the lens from its optimal position and secondly, it mayapply force to the lens and cause it to distort thus reducing itsoptical quality and as a result reduce the radiance of the beam.Furthermore, a small NA may result in reflections towards the laser,which might interfere with the laser mode and further reduce theoriginal beam's radiance, which may further harm the radiance of theemitted beam. The light emitted from the edges of the lens, if a smallNA lens is used, may interfere with the operation of other parts of thesystem, such as beam monitors, a tracking servo or other opticalelements in the system, or may cause excessive heating to other portionsof the system, which may interfere with their operation.

Reference is now made to FIG. 26 showing a block diagram of the laserprotector 251. As mentioned above, safety system 31 assesses thepotential for a safety breach and notifies controller 13 in case suchpotential exceeds a threshold. Controller 13 may then command laserdriver 12 to terminate or reduce the power supplied to laser 252, whichmay be the laser which is emitting beam 15, or it may be the laserpumping the gain medium which is used to generate beam 15. Suchtermination of power may need to be very fast. If the power supplied iscut or reduced suddenly, negative voltages may develop in the conductorscarrying the laser driver current (if those are electrical conductors),which may damage laser 252. To prevent such damage to the laser 252,laser protector 251 is connected between laser driver 12 and the laser252, typically close to the laser 252. Laser protector 251 protects thelaser 252 from negative voltages, typically by connecting a diode, or anequivalent circuit/component such as a Zener diode, a varistor or acircuit designed to drain such excessive negative voltage quickly,between the current conductors, such that when a negative voltage existsbetween the conductors, current flows through the protective diode orequivalent circuit, causing a fast decay of the voltage to safe levels.Laser protector 251 can also be used to protect the laser fromoverheating, or from current waves, by attenuating the power sent tolaser 252 when over-temperature or overcurrent is sensed.

It is appreciated by persons skilled in the art that the presentinvention is not limited by what has been particularly shown anddescribed hereinabove. Rather the scope of the present inventionincludes both combinations and subcombinations of various featuresdescribed hereinabove as well as variations and modifications theretowhich would occur to a person of skill in the art upon reading the abovedescription and which are not in the prior art.

1. A system for optical wireless power transmission to at least onepower receiving apparatus, said system comprising: an optical resonatorhaving end reflectors and a gain medium, said gain medium comprisingeither (i) a semiconductor material, or (ii) a solid host, doped withNeodymium ions and in optical communication with a filter attenuatingradiation for at least one frequency having a wave number in the range8,300 cm⁻¹ to 12,500 cm⁻¹, and wherein said gain medium: (a) ispositioned inside said optical resonator, (b) has a first bandgapenergy, and (c) is thermally attached to a cooling system, such thatsaid gain medium is configured to amplify light passing through it, andsaid resonator is configured to emit an optical beam of said light; adriver configured to supply power to said gain medium, and enablingcontrol of the small signal gain of said gain medium; a beam steeringapparatus configured to direct said optical beam in at least one of aplurality of directions; an optical-to-electrical power converterlocated in said at least one power receiving apparatus, and configuredto convert said optical beam into electrical power having a voltage,said optical-to-electrical power converter having a second bandgapenergy; a detector configured to provide a signal indicative of saidoptical beam impinging on said optical-to-electrical power converter;and a controller adapted to control at least one of the status of saidbeam steering apparatus and said driver, said controller receiving acontrol input signal at least from said detector, wherein saidcontroller is configured to respond to an indication of a safety riskoccurring in the system, by outputting a command to result in at leastone of: causing said driver to change the small signal gain of the gainmedium; changing the radiance of said optical beam; changing the powersupplied by said driver; changing the scan speed of said beam steeringapparatus; changing the scan pose of said beam steering apparatus; andrecording the scan pose which defines the location of saidoptical-to-electrical power converter.
 2. The system according to claim1 wherein said indication of a safety risk occurring in the system isobtained at least from said signal generated by said detector configuredto provide a signal indicative of said optical beam impinging on saidoptical-to-electrical power converter, and from a signal generated bythe level received at said resonator of said beam reflected from said atleast one power receiving apparatus.
 3. The system according to claim 1,further including a power sensor disposed such that it provides a signalindicative of the power carried by said optical beam before impingementon said at least one power receiving apparatus.
 4. The system accordingto claim 3 wherein said driver is configured to reduce said small signalgain of said gain medium when said power indication of said power sensorexceeds a threshold.
 5. The system according to claim 3 wherein saiddetector also provides a signal indicative of the power received by saidat least one power receiving apparatus.
 6. The system according to claim5 wherein at least one of said indications of safety comes from adifference between said power indicated by said power sensor and saidpower indicated by said detector in one of said at least one powerreceiving apparatus.
 7. The system according to claim 6 wherein said atleast one of said indications of safety arises from said differenceexceeding a threshold.
 8. The system according to claim 1, furtherincluding a beam penetration sensor adapted to sense when an unwantedobject enters said optical beam, said entry of said unwanted objectconstituting an indication of a safety risk.
 9. The system according toclaim 1, further including an enclosure integrity sensor, wherein awarning issued by said sensor of lack of integrity of said enclosureindicates a safety risk.
 10. The system according to claim 1 furtherincluding a sensing device for sensing a deviant operation of at leastone critical subsystem in said system, said deviant operationconstituting an indication of a safety risk.
 11. The system according toclaim 1, wherein said second bandgap energy is smaller than said firstbandgap energy.
 12. A system for optical wireless power transmission toat least one power receiving apparatus, said system comprising: anoptical resonator having end reflectors and a gain medium, said gainmedium comprising either (i) a semiconductor material, or (ii) a solidhost, doped with Neodymium ions and in optical communication with afilter attenuating radiation for at least one frequency having a wavenumber in the range 8,300 cm⁻¹ to 12,500 cm⁻¹, and wherein said gainmedium: (a) is positioned inside said optical resonator, (b) has a firstbandgap energy, and (c) is thermally attached to a cooling system, suchthat said gain medium is configured to amplify light passing through it,and said resonator is configured to emit an optical beam of said light;a driver configured to supply power to said gain medium, and enablingcontrol of the small signal gain of said gain medium; a beam steeringapparatus configured to direct said optical beam in at least one of aplurality of directions; an optical-to-electrical power converterlocated in said at least one power receiving apparatus, and configuredto convert said optical beam into electrical power having a voltage,said optical-to-electrical power converter having a second bandgapenergy; a detector configured to provide a signal indicative of saidoptical beam impinging on said optical-to-electrical power converter;and a controller adapted to control at least one of the status of saidbeam steering apparatus and said driver, said controller receiving acontrol input signal at least from said detector, such that said opticalbeam has a radiance of at least 8 kW/m²/Steradian, and when saidcontroller directs said optical beam onto one of said power receivingapparatuses, the overall radiance efficiency of transmission is at least20%.
 13. The system according to claim 12, wherein the overall radianceefficiency of said transmission is at least 30%.
 14. The systemaccording to claim 12, wherein said system is configured to maintain theoverall radiance efficiency of transmission across a predetermined rangeof operational temperatures.
 15. The system according to claim 12,further including a radiance detector in said at least one powerreceiving apparatus, providing a signal indicative of the radiancereceived by said power receiving apparatus.
 16. The system according toclaim 15, wherein said overall radiance efficiency is maintained becauseof at least one of (i) constructural parameters of said system, and (ii)the provision of a radiance maintaining feedback system, adapted to usesaid signal indicative said radiance and to adapt system parameters tomaintain it.
 17. The system according to claim 16, wherein said systemparameters include at least one of a lens position, a lens optical powerand said beam power.
 18. The system according to claim 15 furtherincluding a transmission radiance detector disposed to provide a signalindicative of the radiance emitted by said optical wireless powertransmission system.
 19. The system according to claim 18, wherein anindication of a safety risk is triggered when the difference between thesignal from said transmission radiance detector and said signalindicative of the radiance received by said power receiving apparatusexceeds a threshold value.
 20. The system according to claim 12, whereinsaid second bandgap energy is smaller than said first bandgap energy.21. The system according to claim 1, wherein said indication of a safetyrisk occurring in the system is further obtained when the differencebetween a signal indicative of the transmission radiance and a signalindicative of the radiance received by said power receiving apparatusexceeds a threshold value.